That animals, or plants, so minute that a hundred millions of distinct individuals will scarcely weigh a single grain, should form accumulations hundreds of feet in thickness and extending over thousands of square miles, seems a hardly credible statement; but a fact still more difficult to believe and comprehend is one which is thoroughly established by abundant evidence, namely: that immense deposits of volcanic materials, or, at least, of materials closely connected in their origin and nature with volcanic action, and spread over vast tracts of country in different parts of the world, are also, to a large extent, made up of these microscopic organisms, the existence of which seems dependent on the presence of water, and so utterly at variance with a condition of volcanic activity.
Throughout this volcanic region of California, Oregon, Nevada, and probably as far north as the igneous masses extend, which are well known to cover a vast area on the western side of our continent, there are found deposits, which are usually called “fire-clay,” “kaolin,” “pipe-clay,” or simply “clay;”[30] these masses are, however, not at all of the nature of kaolin, nor are they proper clay, although they may, in places, pass into clay or shale.
The material of which this deposit is made up is exceedingly fine-grained, seemingly an impalpable powder, usually perfectly white and more or less distinctly stratified. It is extremely light, and resembles commercial magnesia more than anything else. In its geological position, it is found underlying the basaltic masses, or the products of the last great eruptive action of the Sierra Nevada. It is often associated with, or intercalated among beds of gravel, fine or coarse-grained sandstone and shales, and bears the evident marks of being a sedimentary deposit made along the sides of a gently-descending broad valley, or lake-like expansion of a valley. This is its character in the Sierra Nevada; but as we go north and northeast, and come on to the great volcanic table lands of Northern California and Southern and Eastern Oregon, we find the thickness of the deposits of this kind of material increasing, and the area occupied by them more considerable. The following localities are especially worthy of notice: North of Virginia City, Nevada; Surprise Valley; Pit River, near mouth of Canoe Creek; Klamath Basin, or in the vicinity of Wright, Rhett and Klamath Lakes; the Des Chutes Basin.
Of all the localities, the last mentioned would seem to be the most remarkable for the extent and thickness of the deposits in question. It was from here that the first specimens examined by Ehrenberg, in 1849, were brought by Frémont, who represented the deposit as 500 feet thick. This region has since been examined by Dr. Newberry, who describes the cañons of the tributaries of the Des Chutes as in places 2,000 feet deep, the plateaux between which cañons are covered by basaltic lava, and this is seen, in the magnificent sections thus presented, to rest on a thickness of hundreds of feet of tufaceous strata interstratified with a variety of beds of volcanic conglomerates, pumice sand, ashes, etc. Dr. Newberry speaks of tufaceous strata 1,200 feet in thickness, in the cañon near the mouth of the Mptolyas River.
The white material, of which some of the more prominent localities have been indicated above, and which is well known to explorers under so many names, as already mentioned, is in reality chiefly of a silicious character, and made up, to a large extent, of organic bodies of microscopic dimensions, infusoria, or Diatomaceæ. This fact was first recognized in the case of the specimens collected by Frémont on the Des Chutes River, and examined by Bailey and Ehrenberg. Specimens collected by Dr. Newberry, on the Pacific Railroad Survey, were also examined by Professor Bailey, but I am not aware that any detailed description of the results was ever published.
Among the collection of the Geological Survey are a large number of specimens of the white infusorial deposit, underlying the lava at various localities. Of these a preliminary examination has been made by Professor Brewer, and a large supply of material is now in the hands of Mr. A. M. Edwards, of New York, for a detailed examination and report. The fact has been already well demonstrated that all or nearly all these fine, white, light masses are made up, to a large extent, of the silicious remains of the diatomaceæ, and in all cases of forms peculiar to fresh water. The geological position of these beds is extremely recent. They extend from the latter portion of the Pliocene into the Post-pliocene epoch, and seem to have continued their existence nearly, if not quite, down to the present day.
So far the facts are very simple, and the principal results of our detailed microscopic examination of these infusorial deposits will be, the knowledge of the range of the different species which occur in them, and the relations of the various forms to those now living, either in this region or in other parts of the world. This the extent of our collections will give us better opportunities to do than others have yet had.
There is a point, however, of great interest connected with these deposits, in regard to which I desire to make some remarks at this present time, and on which I consider that our explorations are capable of throwing some light.
Ehrenberg has recently[31] examined a specimen collected many years ago, in the Toluca Valley, Mexico, by the well-known mining engineer Burkart, of what he denominates a “Phytolitharien Tuff” or phytolithic tufa, and which came to him labeled “Trachytic Tufa, from Toluca Valley, quere, whether pumice-like or infusorial.” Of this, Ehrenberg says: “It is a silver-gray, easily crumbled, gritty tufa, which does not effervesce with acids, and which, when heated, becomes darker, but not black, and then assumes a light-brownish color.” The microscopic analysis of it showed that it was made up to a large extent of phytolitharia, which probably belong chiefly to the grasses, and between them lie scattered a comparatively small number of bacillaria. All are fresh-water forms.