852 PAPERS RELATING TO ANTHROPOLOGY. 



will destroy nearly all other rocks. But there is one way in which it 

 can easily be reduced : the alkaline minerals, such as sodium, potassium, 

 and a few others, form compounds which are highly soluble ; when 

 water, especially if thermal, thus charged, comes in contact with quartz, 

 the rock is easily decomposed and held in solution until, by cooling or 

 evaporation, the water will not retain so much foreign matter, and the 

 silica is deposited. In this manner is the quartz supplied which forms 

 our agates, geodes, crystals, the concretions around hot springs, &c. 



Should the dissolved silica be carried onward, however, to a consid- 

 erable body of water, it tends to produce flint or its allied forms in large 

 masses; and there are different ways in which this can be brought 

 about. In sea-water there exist microscopic animals whose shells are 

 formed from silica in the same manner that the shells and skeletons of 

 mollusks and corals are formed from carbonate of lime; and upon the 

 death of these animals their remains find a resting place upon the bot- 

 tom of the ocean, and hold a limited place in the rocks formed by the 

 limestone-makers — limited because, although numerous, these animal- 

 culse are so small that thousands or even millions of them will not equal 

 in bulk a single clam-shell. 



Generally the flint thus formed is scattered through the limestone in 

 such a waj^ as to be scarcely, if at all, noticeable ; in the Niagara di- 

 vision of the Upper Silurian age it seems first to collect itself in nodules 

 or masses by that mysterious proceeding called " concretionary ac- 

 tion," a something which has never been explained. Even when the 

 flint lies in a regular stratum, as it sometimes does, the layer is not con- 

 tinuous, but is broken up into these concretions. Similar masses are 

 found in the succeeding geological formations, especially in the Chalk- 

 measures ; in fact, some geologists claim that the only true flint is that 

 found in the Cretaceous rocks of Europe, and that flint proper is not 

 found in America at all. Be that as it may, the term is now too firmly 

 attached to our siliceous rocks of this nature ever to be changed. Flint 

 formed thus, however, is found only distributed in other rocks and does 

 not occur in masses to itself. 



The principal ^iw^6eds are found in the Carboniferous age, and the 

 manner in which they are found associated with the other rocks of the 

 series shows another of the methods of flint-making. 



In the lagoons of the present day exist immense numbers of minute 

 plants called Diatoms, which have the power of abstracting silica from 

 the water and using it in their plant structure. On the decay of the 

 plant the silica is not restored to the water, but is precipitated ; and if 

 this work is allowed to continue undisturbed, in time a thick deposit 

 is the result. As a general thing, the slower the process the more com- 

 pact the stone will be ; and if no tides or freshets interfere, beds of pure 

 siliceous rock inches or even feet in thickness may be formed. Geolog- 

 ical conditions at the beginning of the Carboniferous age seem to have 

 been favorable for the existence of these Diatoms in vast numbers, for 



