NATURAL HISTORY OF THE DIATOMACEtE. 4 



the stones composing it, so large and clear, have been rounded where they lie by a 

 current or waves of water. The appearance presented by this bed of boulders is dif- 

 ferent from that of any of the beds of volcanic conglomerate which are so common in 

 many parts of California and Oregon, or of the stratified conglomerates of the Sacra- 

 mento valley, and it is undoubtedly of local origin. The trap which formed the 

 greater part of the bank above is evidently of recent date, more recent than the infu- 

 sorial marls, and the marls more recent than the conglomerate, and the conglomerate 

 an accumulation of rolled stones and pebbles, which belongs to the present epoch. 

 The trap which overlies the infusorial marls composes a large part of the walls of the 

 canon at this point, where it has been cut away by the stream, and forms nearly per- 

 pendicular faces of several hundred feet in height. The soft nature of the underlying 

 strata has, however, very much assisted in its removal (p. 33). 



There are several localities besides those mentioned at which this, 

 what I have chosen to designate "sub-plutonic," material is found, as 

 at Klamath lake, on the northern border of California, and elsewhere all 

 through the Pacific states. From these I have received gatherings, and 

 have thus been enabled to examine, by means of the microscope, spec- 

 imens from many points in what was once this chain of enormous 

 fresh-water inland seas, for such they deserve to be styled. For as the 

 microscope reveals the fact, the organisms, whose stony remains consti- 

 tute the mass of these deposits, were inhabitants of collections of fresh 

 water which existed at some past period as large lakes ; and a careful 

 geographical examination of the country enables us even to indicate, to 

 a certain extent, the situations once occupied by these now extinct seas, 

 which at times varied in superficial dimensions, and certainly were in 

 some cases drained, overflowed by lava, and renewed and replenished 

 with living organisms as many as seven times. 



And now that we understand how it is that lacustrine sedimentary 

 deposits are formed by the accumulation of the dead shells of diato- 

 maceas, we can comprehend the manner in which these sub-plutonic 

 strata have been laid down. If we look at the map of the western 

 coast of the North American continent, we see that there are three 

 great chains of mountains, about parallel to each other and the coast 

 line, and thus enclosing between their peaks two long and wide valleys. 

 The Rocky Mountains are the first of these chains, and they at one 

 time formed the coast of this continent. Slowly and gradually, however, 

 there appeared a line of islands at a distance from the coast, whose 



