ROCKS. 255 



the atmosphere was overcharged, they became fitted to hold 

 iu solution a larger quantity of lime. 



The sedimentary strata, setting aside ail other exogenous, 

 purely mechanical deposits of sand or detritus, are as follows : 



Schist, of the lower and upper transition rock, composing 

 the silurian and devonian formations ; from the lower silurian 

 strata, which were once termed cambrian, to the upper strata 

 of the old red sandstone or devonian formation, immediately 

 in contact with the mountain limestone. 



Carboniferous deposits : 



Limestones imbedded in the transition and carboniferous 

 formations ; zechstein, muschelktilk, Jura formation and chalk, 

 also that portion of the tertiary formation which is not includ- 

 ed in sandstone and conglomerate. 



Travertine, fresh- water limestone, and silicious concretions 

 of hot springs, formations which have not been produced un- 

 der the pressure of a large body of sea water, but almost in 

 immediate contact with the atmosphere, as in shallow marsh- 

 es and streams. 



Infusorial deposits : geognostical phenomena, whose great 

 importance in proving the influence of organic activity in the 

 formation of the solid part of the earth's crust was first dis- 

 covered at a recent period by my highly-gifted friend and fel- 

 low-traveler, Ehrenberg. 



If, in this short and superficial view of the mineral con- 

 stituents of the earth's crust, I do not place immediately after 

 the simple sedimentary rocks the conglomerates and sandstone 

 formations which have also been deposited as sedimentary 

 strata from liquids, and which have been imbedded alternate- 

 ly with schist and limestone, it is only because they contain, 

 together with the detritus of eruptive and sedimentary rocks, 

 also the detritus of gneiss, mica slate, and other metamorphic 

 masses. The obscure process of this metamorphism, and tho 

 action it produces, must therefore compose the third class of 

 the fundamental forms of rock. 



Endogenous or erupted rocks (granite, porphyry, and mela- 

 phyre) produce, as I have already frequently remarked, not 

 only dynamical, shaking, upheaving actions, either vertically 

 or laterally displacing the strata, but they also occasion chang- 

 es in their chemical composition as well as in the nature of 

 their internal structure ; new rocks being thus formed, aa 

 gneiss, mica slate, and granular limestone (Carrara and Pa- 

 rian marble). The old silurian or devonian transition schists, 

 the belemnitic limestone of Tarantaise, and the dull gray cal- 



