438 



GEOLOGY. 



attention. The percolating waters are constantly taking up matter as 

 well as throwing it down, and so, while they are cementing fragments 

 together and heahng fractm^es, they are also removing material, and a 

 rock may be growing porous and cavernous at the same time that its 

 fragments are being united. Cavities may be formed at one stage and 

 filled at another; matter may be taken up at one point and put down at 

 another, and so an internal reconstruction is in slow progress. 



Fig. 360. — Veins of calcite in limestone. Calciferous formation near Highgate 

 Springs, Vt. (Walcott, U. S. Geol. Surv.) 



Concretions. — A notable phase of this internal reconstruction is the 

 assembling together of like kinds of matter. For instance, silica that 

 w^as probably deposited in the form of the silicious shells and spicules 

 of plants and animals, and was disseminated through the sediments as 

 originally formed, is aggregated into nodules of chert or flint (Fig. 361) ; 

 similarly, concretions of ferrous carbonate or calcium carbonate grow in 

 sands, silts, or muds; clusters of crystals of pyrite (FeSo), of sphalerite 

 (ZnS), and galenite (PbS) are formed in clayey layers, pressing the clay 



