In his remarks on this material, Ehrenberg recalls the other specimens of infusorial tufas, which have been examined by him, at various times, since 1839. He mentions particularly the rock from the Des Chutes River, collected by Frémont; also trachytic tufa, with organic remains, from Honduras; trachytic tufa from the volcano Maibu, in Chile; the mud-ejections (?) of the volcanoes near Quito; the ejections (?) of the volcano Imbabaru, as well as those from the island of Guadaloupe.
In regard to the Des Chutes River deposit, it may be incidentally remarked that the eminent microscopist seems to assign to it a much greater geological age than it really deserves; it is, unquestionably, as recent as the latter part of the Pliocene.
It would appear from what Ehrenberg has published, that he considers this occurrence of organic forms, in connection with reputed volcanic masses, to be something extremely difficult to explain, as indeed it is, if we adopt the view taken by him, namely, that these so-called tufaceous materials are the direct products of volcanic action; that is to say, that they have been ejected from craters, either in the form of showers of ashes or of mud out-flows. It would be, indeed, to my comprehension, something entirely inexplicable, that such vast masses of matter, made up to a large extent of organic forms, should be poured forth from the interior of the earth. This would be the case, as it appears to me, no matter what theory of volcanic action one might choose to adopt; since, whatever may be the cause, no one will deny that a high temperature is, at least, one of the results. That Ehrenberg really considers these infusorial deposits to be of eruptive origin, is evident from a remark in his last communication, (that in reference to the specimen from the Toluca Valley) to the effect that the occurrence of fresh-water forms, exclusively, in these infusorial masses is evidence that volcanic phenomena are not dependent on, or connected with, the presence of sea-water, as is generally supposed, from the fact that volcanoes are situated, in most cases, near the sea coast.
Not having the necessary works of reference at hand to be able to see, in all the cases cited by Ehrenberg, exactly what the evidence is, on which his theory of the origin of these infusorial deposits is founded, I will not attempt to give an authoritative statement in regard to any others than those which belong to this coast; but I cannot avoid drawing the inference, that the same conditions which are so easily traced here will, on future examination, be found existing in all the other localities cited by him.
The mode of occurrence of these fresh-water infusorial deposits in California, and on the Pacific coast in general, is very simple. They are accumulations of organisms which have been collected at the bottom of the lakes, or in the lake-like shallow expansions of rivers, in which they grew. This growth took place at a time when volcanic agencies were busily at work, giving rise to accumulations of ashes, pumice, and other materials. The rapidity with which these infusorial deposits form, at the present time even, the vast extent over which they are distributed, and the general importance in the geological history of the earth, are now matters which are well understood, of the masses thus accumulated and in regard to which the store of facts has been rapidly growing in magnitude during the past few years. The mud deposits and deltas of rivers, the bottoms of lakes and swamps, and the bed of the ocean itself, are the repositories of these forms. Heat and stagnant water seem to be what is required for their rapid reproduction and the consequent rapid accumulation of their remains.
The infusorial deposits of Central California—I refer now to those of fresh water origin, and connected with volcanic masses—are all situated in such positions as to show, that they were formed and deposited in shallow water; that, through the various alternations of calm and convulsion in the Sierra, they were at one time allowed to accumulate in quiet, then swept over by masses of gravel and sand, indicating a furious rush of water, then covered with a shower of ashes and pumice from the neighboring volcanoes of the Sierra then in active operation; and finally, at the grand finale of the basaltic lava overflow of the chain, capped with this indestructible material, which has effectually prevented the washing away of the otherwise easily removed infusorial deposits. This is the connection between the volcanic and the infusorial masses; by their absolute indestructibility the former have protected the latter from denudation, and consequently we see them always accompanying each other: for where the cover did not exist, there the denuding forces have swept away every vestige of the soft and easily yielding material, or else it remains concealed under the water. To form an idea of the extent of the erosion which has taken place since these infusorial beds were deposited, and the consequent change in the configuration of the country, we must bear in mind that the whole of the present river cañons on the west slope of the Sierra have been excavated since that time, and that, in many places, the strata have been removed to a vertical depth of between two and three thousand feet.
Everything shows that the surface covered by fresh water in the region east of the crest of the Sierra was, at a not very distant epoch, much greater in extent than it now is. There existed, probably during or immediately after the glacial epoch, a chain of great lakes occupying a large portion of the country from Walker’s Lake to the Des Chutes River, a distance of about four hundred miles, and extending over a breadth of not less than one hundred. A large portion of this region is now a volcanic plateau; and, where cut into by the force of running water, the deposits of infusorial strata may be seen, sometimes thin and unimportant, but often of great thickness. Observations and measurements of terraces and determination of the altitude of all these old lake deposits will enable us at some future time to indicate on the map the area once occupied by this great chain of inland seas. The vast extent of the lacustrine infusorial formations on the east side of the Sierra is thus accounted for, as well as the comparatively small area which they cover on the western slope.
In addition to the stratigraphical reason given above why the infusorial strata should occur connected with eruptive masses, there may be a chemical one which shall, in part, account for the apparent great development of the diatomaceæ in volcanic regions. These organisms require an amount of silica, infinitesimally small for each individual, but in reality enormous for the number of organisms required to develop themselves over the vast area and with the thickness which they occupy. That a volcanic region should supply a larger amount of silica in the state in which it can be appropriated by the diatomaceæ, is extremely probable. We know that silicification of all organic matters occurring in these volcanic regions of our coast proceeds with the greatest rapidity, and has taken place on an extensive scale. The thermal springs contain a great amount of free silica, and it is in the vicinity of such springs that large infusorial deposits are frequently found. It seems that it could only be in regions particularly favorable for the secretion of their silicious coverings, that these infusoria could be accumulated with such rapidity as to form what may be called, without exaggeration, mountain masses. It is also possible that temperature may have something to do with this rapid development, and that volcanic regions may on this account be favorable to it.
To my apprehension, the phenomena of infusorial deposits in connection with volcanic masses admit of an easy explanation on this coast, at least; and I can hardly believe that any of the localities of diatomaceæ, if closely examined, would present any such difficulties as to make the assumption necessary that they have been ejected from the interior of the earth. In cases where infusoria seem to have been actually ejected from craters, as is said to have been the case in some of the South American volcanoes, it is not difficult to understand that an ancient crater may have become filled up and temporarily converted into a lake; and that, after the growth and deposition of an infusorial deposit at the bottom, a new eruption may have broken out in the same place as a previous one, or in its immediate neighborhood. In such a case, among the ejected material, a large quantity of the infusoria would be found mingled with the ashes, which must pass through the material collected in the bottom of the crater as they rise from the interior of the earth. The bursting of lakes at the bases of volcanic cones, caused by the rapid melting of the snows above them, have often given rise to torrents of volcanic mud, called “Moya” in South America, in which both animal and vegetable remains are often inclosed in great quantity; but the connection between the organic and inorganic phenomena, in such cases, is perfectly evident.
In fact, I see no reason for suspecting any connection between the infusorial deposits and the volcanic masses of this coast, or of any other part of the world, which should influence the geologist in forming an opinion with regard to the cause or the locality of volcanic action.