Mount Rainier Part 18

The bright coloring of the surfaces of the lava blocks and the general appearance of the face of the cliff may indicate fumarole action at this point. There is also some decomposition along the inner edge of the crater rim, near the steam vents. On the lower slopes, some distance below the snow line, the freshness of the rock is not a noticeable feature, and it is seen that here weathering is of the nature of chemical decomposition as well as of mechanical disintegration.

MICROSCOPIC CHARACTERS

Microscopically these lavas show more uniformity than is apparent megascopically. Rocks which in color and texture appear quite diverse are found to be mineralogical equivalents. The majority of these rocks are andesites, the hypersthene-andesites predominating, as was shown by Hague and Iddings; but over large areas the andesites are decidedly basaltic, and, indeed, many of the lavas are basalts. The megascopic differences are mostly referable to groundma.s.s characters, the color of the rock being dependent upon the color and proportion of gla.s.sy base present. Therefore the degree of crystallization of groundma.s.s const.i.tuents is of more importance in determining the megascopic appearance than is the mineralogical composition, and the basaltic lavas are for the most part light gray in color, while the more acid hypersthene-andesites are often black or red.

In petrographic character the lavas range from hypersthene-andesite to basalt. This variation is dependent upon the ferromagnesian silicates, and four rock types are represented--hypersthene-andesite, pyroxene-andesite, augite-andesite, and basalt--any of which may carry small amounts of hornblende. A rigid separation of these rock types, however, is impossible, since insensible gradations connect the most acid with the most basic. In the same flow hypersthene-andesite may occur in one portion, while in close proximity the lava is an augite-andesite.

These lavas have groundma.s.s textures that vary from almost holo-crystalline to gla.s.sy. The felted or hyalopilitic texture is the most common, and plagioclase is the princ.i.p.al groundma.s.s const.i.tuent.

The feldspars are lath-shaped, often with castellated terminations. In the more basic phases anhedrons of augite and of olivine appear, and magnet.i.te grains are usually present. Flowage is often beautifully expressed by the arrangement of the slender laths of feldspar.

Among the phenocrysts feldspar is the most prominent. It has the usual twinning characteristic of plagioclase and belongs to the andesine-labradorite series, extinction angles proving basic andesine and acid labradorite to be the most common. Zonal structure is characteristic, being noticeable even without the use of polarized light. Zonal arrangement of gla.s.s inclusions testifies to the vicissitudes of crystallization, and often the core of a feldspar phenocryst is seen to have suffered corrosion by the magma and subsequently to have been repaired with a zone of feldspar more acid in composition.

Of the darker phenocrysts, the pyroxenes are more abundant than the olivine or hornblende. Hypersthene and augite occur alone or together, and are readily distinguished by their different crystallographic habits as well as by their optical properties. The hypersthene is usually more perfectly idiomorphic and occurs in long prisms, with the pinacoidal planes best developed, while the augite is in stout prisms, usually twinned. Both are light colored, and the pleochroism of the hypersthene is sometimes quite faint. According to the relative importance of these two pyroxenes, the lavas belong to different types, hypersthene-andesite, pyroxene-andesite, or augite-andesite.

Olivine occurs in certain of the Rainier lavas, in stout prisms somewhat rounded and often with reddened borders. The usual a.s.sociation with apat.i.te and magnet.i.te crystals is noted. The olivine varies much in relative abundance, so as to be considered now an accessory and now an essential const.i.tuent, and in the latter case the rock is a basalt.

Hornblende is not abundant in any of the rocks studied, although typical hornblende-andesite has been described among the specimens collected by Professor Zittel. Where it occurs it is in brown crystals, which have usually suffered magmatic alteration. In one case, where this alteration is less marked, the idiomorphic hornblende is found to inclose a crystal of labradorite, and thus must have been one of the latest phenocrysts to crystallize. It also surrounds olivine in this same rock,[29] which is a hypersthene-andesite, the hornblende and olivine being only accessory.

The different textures of these lavas are doubtless expressive primarily of diversity in the physical conditions of consolidation, but also in part of variations in chemical composition. The variations in mineralogical composition are likewise referable to these two factors, but here the latter is the more important. The hypersthene-augite olivine variation, already referred to, doubtless well expresses the chemical composition of the magma, and deserves to be taken as the chief criterion in the cla.s.sification of the lavas. As was noted by Hague and Iddings, the hypersthene and olivine play a like role, the former occurring when the silica percentage is somewhat higher than in basalt. It is exceptional to find the two in the same specimen, the one being absent whenever the other is present. The following a.n.a.lysis[30] of the typical hypersthene-andesite from Crater Peak shows the lava to be a comparatively acid andesite:

a.n.a.lYSIS OF HYPERSTHENE-ANDESITE FROM CRATER PEAK, MOUNT RAINIER

PER CENT.

SiO_{2} 61.62 Al_{2}O_{3} 16.86 FeO 6.61 CaO 6.57 MgO 2.17 Na_{2}O 3.93 K_{2}O 1.66 ----- 99.42

An a.n.a.lysis[31] of one of the light-gray, olivine-bearing rocks on the northern slope of the mountain gives a silica percentage of 54.86, and is doubtless representative of the more basic of the Rainier lavas.

The sporadic occurrence of hornblende in these andesites is princ.i.p.ally the result of physical conditions rather than of chemical composition. The magmatic alteration of the phenocrysts of hornblende affords evidence of this variation in consolidation conditions, a diminution of pressure with continuance of slow cooling giving rise to the magmatic alteration of the hornblende. That this change took place during the later stages of consolidation is shown by the relative age of the hornblende, noted above, and also by the fact that in one case a phenocryst of augite, where it abuts against the hornblende, has protected the latter from this alteration. The alteration is in part pseudomorphic, the hornblende retaining its characteristic outlines, but often there has been resorption. In one andesite the abundance of these remnants of hornblende and also of augite anhedrons in the groundma.s.s may justify the conclusion that this augite andesite is of derivative origin, of the cla.s.s described by Was.h.i.+ngton.[32] It may be noted also that hypersthene shows a tendency to magmatic alteration, although only rarely.

In a basal flow in Moraine Park, the slaggy and compact phases show differences in phenocrysts as well as in groundma.s.s. The gla.s.sy rock has hypersthene as the predominant phenocryst, while feldspar is the more important in the compact and more crystalline andesite.

The distribution of the rock types described above is of interest. On the northern slope of the mountain, between Willis and Carbon glaciers, the characteristic lava is a gray andesite, smooth to rough in texture, and showing platy and columnar parting. Hypersthene is not the prevailing pyroxene, and olivine is usually present, often in such abundance as to make the rock a basalt.

In Moraine Park gray andesites also predominate, with both pyroxenes as phenocrysts, but here hypersthene is the more important. On the eastern slope on the Wedge, between Winthrop and Emmons glaciers, the lavas are pyroxene-andesites and vary much in megascopic appearance, although little in microscopic characters. These rocks are quite distinct from any seen to the north. The nunatak in Emmons Glacier is composed of hypersthene-andesite, but on Little Tahoma the lava shows more variety. Both augite-andesite and hypersthene-andesite occur, while at the southern end of this interglacial rock ma.s.s, just east of Cowlitz Glacier, the cliffs are composed of the prismatic black basalt. On Crater Peak, and below on Gibraltar, hypersthene andesite occurs with considerable variation of color and texture. On the spurs west of Nisqually Glacier the andesites contain both pyroxenes, the augite being somewhat the more important.

The distribution of the volcanic rocks, as determined in the study of reconnaissance collections, indicates that the cone has been built up by eruptions of lava and of fragmental material. The successive lava streams were doubtless of considerable thickness, but were limited in lateral extent. The beds of fragmental material are of the nature of flow breccias and of coa.r.s.e agglomerates on the higher slopes, while tuffs occur at a greater distance from the center of eruption. This composite cone appears to be remarkably free from radial dikes, which may indicate that the volcanic energy was expended chiefly at the crater. The variation in rock types on different sides of the volcanic cone may be evidence of changes in position of the center of eruption.

The destruction of an earlier crater and the eccentric position of a later would give rise to such a radial distribution of lavas as has been described above.

GRANITE

OCCURRENCE

The presence of an acid holocrystalline rock on the slopes of Mount Rainier was first reported by Lieutenant Kautz in 1857, from whose accounts Dr. George Gibbs was led to announce the occurrence of granite as a dike in recent lavas.[33] Emmons in 1870 observed a cliff of "beautiful white syenitic granite" rising above the foot of Nisqually Glacier and correctly interpreted the geologic relations. In 1895, on a reconnaissance trip, the writer identified granite among the bowlders composing the lateral moraines of Carbon Glacier, as well as on the surface of the glacier itself, and in the following season bowlders of granite were found to be plentiful in the river bed at the foot of this glacier. This anomaly of granite bowlders coming from a volcanic peak was also noted in the canyon of the Nisqually by Emmons.

In the somewhat more careful study of the Mount Rainier rocks, search was made and the granite was found in place at several points on the northeastern slope. A biot.i.te-hornblende-granite was observed on Carbon River at the mouth of Canada Creek, about 12 miles from the summit of Mount Rainier, and at Chenuis Falls, 2 miles up the river, a finer grained holocrystalline rock occurs, apparently an aplitic phase of the granite. In the lower portion of Carbon Glacier, near its eastern edge, a nunatak of granite can be seen, while the same rock occurs farther to the east, beyond the older of the lateral moraines.

Higher on the slopes of Rainier a more marked ridge of granite was traced. A k.n.o.b rises above the eastern moraine of Carbon Glacier at an alt.i.tude of between 7,000 and 8,000 feet, and the more prominent features to the east in Moraine Park also owe their survival to the greater erosion-resisting power of the granite.

PETROGRAPHIC DESCRIPTION

These granites have few features worthy of special mention. Hornblende and biot.i.te are the ferromagnesian const.i.tuents and vary much in relative importance. The variations from hornblende-granite to biot.i.te-granite occur in the same k.n.o.b or ridge, and considering all occurrences the two varieties seem to be of equal development. There is also some variation in the amount of quartz present, and in the relative importance of the orthoclase and plagioclase. All of these characters are also found in the granites of the Northern Cascades.

RELATION TO THE VOLCANIC ROCKS

Along the side of the k.n.o.b overlooking Carbon Glacier the granite as seen from a distance appears to be intrusive. Blocks of andesite cover the slope, deposited there by the glacier at a time when it possessed greater lateral extent, and the granite talus from above crosses this same slope in a narrow band. The relations prove less deceptive on close examination, and the granite is seen to const.i.tute an older ridge. Farther along this ridge, at the cliffs on the north-eastern edge of Moraine Park, the granitic rock is found over-lain by the lava. The actual contact of the two rocks is concealed by soil filling the crevice left by disintegration along the contact plane. The granite, however, exhibits no intrusive characters, while the overlying andesite becomes scoriaceous in its lower portion, although compact immediately above. This contact is on the southern side of the granite ridge, the crest of which is approximately east-west. This position of the lava contact considerably below the highest occurrence of the granite indicates that the topographic features of this old granite ridge were even more marked at the time of the eruption of the lavas and the building of the volcanic cone. Above this ridge of granite on the one side tower the cliffs of bedded volcanics which compose the Sluiskin Mountains, and on the other is the andesite ridge bounding the canyon of Winthrop Glacier. Thus Mount Rainier, although a volcanic peak, rests upon an elevated platform of granite which is exposed by erosion at a few points on the slopes of the mountain.

SUMMARY

The volcanic rocks of Mount Rainier include both lavas and pyroclastics. The breccias, agglomerates, and tuffs, although of striking appearance, are, perhaps, less important elements in the construction of the composite cone.

The lavas vary much in color and texture, but these megascopic differences are referable rather to the degree of crystallization of the magma than to its chemical character. The variation in the chemical composition of the lavas expresses itself in mineralogical differences, and thus four rock types are distinguished--hypersthene-andesite, pyroxene-andesite, augite-andesite, and basalt. The distribution of these types indicates a radial arrangement of lava streams, and hypersthene-andesite is the more abundant variety of lava.

Granite is exposed on the slopes of Rainier where erosion has cut away the overlying lava, and it is plain that the volcanic cone rests upon an elevated platform of older rock, approximately 8,000 feet above sea level.

[Ill.u.s.tration: _Copyright by Harris & Ewing, Was.h.i.+ngton, D. C._ PROFESSOR CHARLES VANCOUVER PIPER]

XVI. THE FLORA OF MOUNT RAINIER

BY PROFESSOR CHARLES V. PIPER

Charles Vancouver Piper was born on Vancouver Island, at Victoria, British Columbia, on June 16, 1867. He graduated from the University of Was.h.i.+ngton in 1885 and since then has received degrees and honors from other inst.i.tutions and learned societies. He was professor of botany and zoology at the Was.h.i.+ngton Agricultural College (now State College of Was.h.i.+ngton) from 1892 to 1903. He has been agrostologist in charge of forage crop investigations for the Bureau of Plant Industry, United States Department of Agriculture, since 1903.

He has discovered many new forms of plant life and has published many monographs and books in the field of botany.

This account of the flora of Mount Rainier was first published in The Mazama (Portland, Oregon) in two articles, one in Volume II, Number 2 (April, 1901), and the other in Volume II, Number 4 (December, 1905). They are reproduced with the consent of the editor of The Mazama, and Professor Piper has revised and amplified them for this purpose.

Up to an elevation of 4,000 feet or more the flanks of Mount Rainier are clothed in a continuous belt of somber forest, broken only where glaciers and their nascent streams have hewn pathways, or where, alas, fire has left desolate slopes marked here and there by the whitened, weather-worn shaft of some old tree, a dreary monument to its destroyed fellows. This forest is composed in its lower reaches largely of Douglas spruce. Scattered through it in smaller quant.i.ties one finds Lovely fir, Western white pine, Western hemlock, a few Engelmann spruces, and on the stream banks cedar and yew, and now and then a little cottonwood.

At about the 3,500-foot level the character of the forest changes. The Western hemlock gives way to the larger-coned Black hemlock; the Douglas spruce and Lovely fir are replaced by the n.o.ble fir; and the ragged-barked Alaska cedar greets the eye. Another thousand feet and the Subalpine fir replaces its two near relatives. From this point upward, the forest, now composed only of Black hemlock, Alaska cedar and Subalpine fir, to which in some places the White-bark pine must be added, is confined largely to the crests of ridges and straggles up the mountain in irregular broken lines. Between these timbered ridges extensive gra.s.sy slopes appear, veritable flower gardens when in their glory.

At 6,500 feet elevation the timber ceases to be. Scraggly prostrate firs and hemlocks, sprawling as it were on the earth for shelter, mark sharply the limit of their endurance. Here, too, the continuous carpet of gra.s.s and flowers ceases--and a soil of volcanic sand or powdered pumice supports a very different vegetation. At 10,000 feet the toughest mountaineer of all the flowering plants, _Smelowskia ovalis_, still appears. Far above this, however, even to the crater's rim, lichens trace their hieroglyphics on the rocks; and on the steam-warmed rocks of the crater two mosses find lodgment, _Hypnum elegans_ Hooker?, and _Philonotis fontana_ Bridel, the latter even in fruit.

Few plants grow in the dense shades of the lower forests, and these are mainly ericaceous. Most plentiful are _Vaccinium ovalifolium_, _V.

macrophyllum_, _Gaultheria ovatifolia_, _Menziesia ferruginea_, _Pachystima myrsinites_, _Cornus canadensis_ and _Clintonia uniflora_.

Here, too, occur several weird-looking whitish or reddish saprophytes, _Monotropa hypopitys_, _Pterospora andromedea_, and _Corallorhiza mertensiana_.

On the drier portions of the gra.s.sy slopes _Lupinus subalpinus_, _Castilleja oreopola_, _Potentilla flabellifolia_, _Pulsatilla occidentalis_, _Erigeron salsuginosus_, _Polygonum bistortoides_, _Phyllodoce empetriformis_, _Ca.s.siope mertensiana_ and _Vaccinium deliciosum_ are the most attractive plants. Where the ground is springy _Veratrum viride_ occurs in great clumps and _Dodecatheon jeffreyi_, _Caltha leptosepala_ and _Ranunculus suksdorfii_ are plentiful.

In the shelter of the Alpine trees _Rhododendron albiflorum_, _Ribes howellii_ and _Arnica latifolia_ flourish. Along the rills _Gentiana calycosa_, _Arnica chamissonis_ and _Mimulus lewisii_ form banks of color. On the cliffs _Chelone nemorosa_, _Spiraea densiflora_, _Polemonium humile_ and _Castilleja rupicola_ are perhaps most conspicuous.

Above the limit of trees, in what have been called "pumice fields," a characteristic series of plants appears. This belt ranges in alt.i.tude from 6,500 to 10,000 feet. It is best developed on the east side of the mountain, where the avalanches from Little Tahoma have covered great areas with more or less finely divided basalt. Conspicuous plants of this region are _Lupinus lyallii_, _Spraguea multiceps_, _Polemonium elegans_, _Hulsea nana_, _Erigeron aureus_, _Oreostemma alpigena_, _Polygonum newberryi_, _Poa suksdorfii_, _Draba aureola_ and _Smelowskia ovalis_. The last three ascend to above Camp Muir, alt.i.tude 10,000 feet.

The first botanist to visit Mount Rainier was Dr. William F. Tolmie, surgeon of the Hudson's Bay Company, who reached the mountain in 1833.

He made considerable collections, which were sent to Sir William Hooker. Among Tolmie's plants were several not previously known.