PREPARATION OF THE DIATOMACE^. 139 



formation in which the Diatomacese appear, has its forms, one and 

 all, extant on our coasts, and I think I am quite correct in stating 

 that every known Diatom, with the exception of one or two 

 doubtful cases, is proved to be still occupying a place on some 

 part of the earth's surface. 



As the fossil deposits are, I think, the most extensive and 

 beautiful in their variety of forms, and are also easily obtainable, 

 they shall receive our first attention. 



Under this head, let us consider the enormous sub-plutonic 

 strata found on the Pacific coast of North America, and described 

 by Professor Meade Edwards, and from which we learn that the 

 fossil deposits may contain both fresh-water and marine species, 

 though never of course in a mixed condition. Marine deposits 

 decidedly predominate, and extend over a very considerable 

 portion of the earth's surface, while fresh-water deposits, though 

 of greater depth, are more decidedly local in their distribution. 

 The Miocene-tertiary age furnishes us with the most important 

 examples, and concerning these a correspondent in Virginia, 

 U.S.A., thus writes me : — " There is a very long chain of 

 Diatomaceous deposits within a few miles of the Atlantic coast, 

 extending from Richmond Va. and Petersburg through Maryland 

 into Pennsylvania ; then follows a chain of fresh-water deposits 

 beginning at Drakeville, New Jersey, extending to Montecello, 

 New York ; then through Connecticut, Rhode Island, Maine, 

 Massachusetts, New Hampshire, and Vermont, into Canada and 

 Nova Scotia. In the first two of these, Eunotia in many species 

 abounds, in the others, varieties of Navicular as N. firma^ JV. 

 rhouiboides, JV. iwiiescens^ and N. serians, but a certain similarity 

 runs through them all, though the gradual appearance and 

 disappearance of certain forms, from ten to twenty miles apart, is 

 very interesting." 



The appearance of the deposit is subject to the following 

 variations, according to Professor Edwards, from a pure white 

 through the various shades of grey, cream, and fawn, to an iron- 

 rust tint. The texture is frequently friable, not unlike clay in its 

 appearance when wet; at other times it is hard and stony, though 

 always porous, and, when soft, of very little weight. As this 

 deposit furnishes us with some of the most lovely known forms, 



