460 PHYSICAL GEOGRAPHY. 



1 87 1. (Proceedings Lyc. Nat. Hist. A T . Y., vol. I, p. 224.) Therein it is 

 shown that gnano is most likely not the excrements of birds or other 

 similar animals, deposited upon the islands and main land after their 

 upheaval, but that it is the result of the accumulation of the bodies of 

 animals and plants, for the most part minute, the diatomaceae making up 

 a large part of the mass, and subsequently upheaved from the bottom 

 of the ocean by volcanic agency, which is known to be very active and 

 pretty constant in that part of the world. In this way guano has 

 become a storehouse of many otherwise rare and beautiful forms of 

 diatomaceae, which can be procured from it by employing a proper pro- 

 cess with chemicals to destroy and remove everything but the siliceous 

 skeletons, which are then left in all their purity, so that their forms 

 may be viewed by means of the microscope. The process for cleaning 

 guano, so as to obtain the microscopic organisms contained in it, will 

 be described hereafter. 



In a semi-fossil condition, the diatomaceae are to be found in all parts 

 of the world, and very extensively within the state of New Hampshire 

 in the form of what have been called lacustrine sedimentary deposits, 

 that is to say, collections of their dead skeletons formed at the bottom 

 of lakes, and going commonly by the name of "marl," although true marl 

 contains few, if any, diatoms, and is largely made up of the shells of 

 mollusca, snails, and the like. The mode of formation of these deposits 

 will be described hereafter. 



Still more ancient, and, what may be with propriety termed truly fossil 

 deposits of fresh-water diatomaceae, are not found on this coast of the 

 North American continent, but, in fact, appear to be confined to the 

 Pacific states, where they cover vast tracts of country. Their mode of 

 formation will be described when we come to treat of the application of 

 a knowledge of the diatomaceae to geology, in a subsequent part of this 

 sketch. Thus extensive strata of diatomaceae have accumulated and 

 become fossilized, and constitute the "infusorial earths" of geologists 

 and others, many of those on our Pacific coast, as has been said, being 

 made up of the remains of fresh-water species which have lived, grown, 

 died, and been laid up in countless millions in the beds of now extinct 

 lakes ; while, likewise, in California, as well as in Virginia and Maryland, 

 in Peru, Japan, and Algeria, are found layers which are made up of the 



