526 THE POPULAR SCIENCE MONTHLY. 



ates, leaving behind heaps of kaolin clays, such as now form, for the 

 same reason, from the decomposition of granite. This reaction is one 

 peculiar to dry laud, and would therefore be subsequent in time to the 

 changes already mentioned. Now, the potassium and sodium carbon- 

 ates, when brought into contact with calcium chlorides, change their 

 composition, and there result calcium carbonates and sodium and po- 

 tassium chlorides. These carbonates, being insoluble, will be precipi- 

 tated to the bottom, and thus will be formed the primitive travertines 

 and limestones, while the sodium chlorides remain in solution to this 

 day, save what has been converted into beds of rock-salt. 



With the removal of the bulk of the acids and possible volatile 

 compounds from the atmosphere, only carbonic acid would remain to 

 render it impure at the close of the era of chemical changes. In later 

 periods this part of the atmosphere has also been removed. The world 

 is not yet ready for life, as there must be further chemical and me- 

 chanical changes. 



The Formation of Sediments. — The next era brings into play a 

 phase of action destined to be the chief agent of change in the world — 

 the erosion of existing ledges to form new rocks. The era opens with 

 a continuation of the atmospheric decompositions, whereby we find 

 silica and alumina remaining in irregular heaps of sand and clay, and 

 the accumulation of calcareous deposits beneath the ocean. 



The formation of thick deposits of inorganic limestone is extremely 

 interesting. Scientists have been wont to ignore altogether the exist- 

 ence of any deposit of this character, since microscopic researches into 

 the structure of many of the calcareous masses exposed at the surface 

 indicate an organic origin. So many shells and coral fragments aid in 

 building up fossiliferous limestone that its mode of growth is very clear. 

 But, after one has spent months in searching vainly for traces of organ- 

 isms among the marble layers of Western Vermont, or the auroral lime- 

 stones of Eastern Pennsylvania, he is tempted to suspect that some of 

 the Silurian limestones even were chemical deposits, though wanting 

 the concentric structure of stalagmite and travertine. But, barring 

 these, and the calcareous dikes in the Laurentian of Northern New 

 York, and in the Silurian beds of Northern Vermont, all the phenom- 

 ena are best explained by the presence of an inorganic limestone be- 

 fore the origin of life. Whence came the materials for the stony 

 habitations of marine animals ? There must have existed great masses 

 of the crude material, stored up in the rocks and in the waters of the 

 sea, to provide with coverings all the testacea of every age, and to 

 furnish the thousands of feet thickness of the Eozoic, Paleozoic, and 

 Mesozoic limestones. This primitive source of supply is now con- 

 cealed, but much of its material has been used over and over again. 



We have suggested how three of the principal rock-materials have 

 been formed — the quartz, clay, and limestone. We have them yet as 

 rude piles of rubbish, neither arranged in layers nor possessing any 



