NATURAL HISTORY OF THE DIATOMACE/E. 467 



colored ones arc diatomaceous. Besides, this material is of a somewhat 

 hard, stony character, but porous withal, and light J as a general thing, 

 also, it is readily broken, but not easily powdered, as are the lacustrine 

 sedimentary deposits. On account of this hardness there is found to be 

 considerable difficulty in preparing these specimens for microscopical 

 examination. After so preparing, by a method I have devised, to be 

 described hereafter, and viewing with a sufficiently high magnifying lens, 

 this substance is found to be made up entirely of the siliceous remains 

 of fresh-water diatomaceae which have been matted together in the re- 

 markable manner described. The species of diatomaceae present, how- 

 ever, are found to be very different in character from those to be seen in 

 the other class of recently formed deposits. Thus, while the genera 

 most commonly represented in and making up the mass of the lacustrine 

 sedimentary deposits are Navkula, Pinnularia, Stauroneis, Synedra, and 

 similar elongated forms, the hard, white material is in general found to 

 consist of myriads of examples of OvtJiosim, Cyclotclla, and similar dis- 

 coid forms. Although our knowledge of the forms of these minute 

 organisms, peculiar to different kinds of collections of water, is rather 

 imperfect, yet we know that the naviculxform genera spoken of above 

 arc found in small lakes, while in the larger pieces of water are to be 

 seen growing more particularly the discoid genera like Cyclotclla. From 

 this fact alone, then, we should be prepared to assume that the waters, 

 in which the organisms whose remains make up these deposits grew at 

 one time, covered large tracts of country. And our surmises on this 

 point are confirmed by the reports of explorers who have passed over 

 this section of country, that is to say, on both sides of the Sierra Ne- 

 vada Mountains, from Puget's sound to the southernmost border of 

 California, for they tell us that these deposits extend over a consider- 

 able portion of the Pacific states. 



I have examined many specimens from this district, and, on account 

 of the mode of occurrence of this material, being capped by lava, basalt, 

 or some volcanically-eruptcd rock, I have designated them sub-plutonic. 

 The first specimens of such sub-plutonic deposits of diatomacea:, which 

 were put into the hands of scientists, were undoubtedly those brought 

 home by Fremont, from his expeditions to the Rocky Mountains, in 

 the year 1842, and to Oregon and North California, in the years 1843 



