3t THE MODERN PERIOD. 



than lialf, of tlic wliolc number. The mud forming in the bottoms 

 of those hikes must contain large quantities of the remains of fresh- 

 water fishes, shell-fish, and other animals, as well as of terrestrial 

 quadrupeds that have been drowned in them or killed on their 

 margins ; and should these lakes be artificially drained, such remains 

 may excite much interest. At present, however, I shall refer to 

 only one kind of lake deposit, which is curious as an evidence 

 of the large quantity of matter that may be accumulated by the 

 growth and death of successive generations of creatures too small 

 to be observed individually except by the microscope. This is 

 the substance known to naturalists as Infusorial earth, and which 

 has been found to abound not only in the deposits from modern 

 waters, but in some ancient rocks, of which it appears indeed 

 sometimes to form the mass. It is, as found in Nova Scotia, a 

 white and, when dry, very light friable earth, having a floury 

 texture, and showing, when examined in a bright light, an infinity 

 of minute shining specks. A little of it diffused in a drop of water, 

 and viewed through a powerful microscope, presents thousands of 

 curiously formed cylindrical, bow-shaped, and rounded transparent 

 bodies, which consist of pure silica or flint, and are the coatings 

 which strengthened the cell-walls of certain minute organisms at 

 one time regarded as animals, but now as one-celled plants of the 

 family Diatomacece. They grow in the waters of some of our 

 lakes in such numbers that their indestructible silicious coverings, 

 in the course of time, accumulate in layers several feet in thickness. 

 The hardness, sharpness, and minute size of these shells render 

 the mass composed of them useful as a polishing material; the 

 best tripoli being, in fact, an earth of this description. The only 

 specimens of this infusorial earth in my possession, and found in 

 Nova Scotia, are from lakes in the hills of Earlton and Cornwallis. 

 That from the last-named locality is the finer of the two. It was 

 discovered by Dr Webster of Kentville. The late Professor Bailey 

 of "West Point, the well-known microscopist, to whom I forwarded 

 specimens from one of the above-named localities, states* that 

 the species contained in it are common to Nova Scotia and the 

 northern parts of the United States. He mentions the following 

 as occun-ing in specimens from Nova Scotia: — Pinnularia viridis, 

 P. Dimqualis, Cocconema ci/mbeforme, GalUonella distans, Eunotia 

 monodon, etc., Ilimantidium arcus, Gomphonema acuminatum, Sur- 

 irclla splendlda, Stauroneis Bayleii ; Spongiolites, etc. Some of 

 these species are represented in Fig. 8. 



* Silliman's Journal, vol. xlviii. 



