HOCKS. 255 



this point of view, a plutonic action seems to a certain extent 

 also to have taken place in the sedimentary strata, especially 

 the more ancient; but these strata appear to have been 

 hardened into a schistose structure, and under great pressure, 

 and not to have been solidified by cooling, like the rocks that 

 have issued from the interior, as for instance granite, porphyry, 

 and basalt. By degrees, as the waters lost their temperature, 

 and were able to absorb a copious supply of the carbonic acid 

 gas with which the atmosphere was overcharged, they become 

 fitted to hold in solution a larger quantity of lime. 



The sedimentary strata, setting aside all 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. muschelkalk, Jura formation and chalk, 

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

 eluded in sandstone and conglomerate. 



Travertine, fresh- water limestone, o.d siliceous concretiont 

 of hot springs, formations which have not been produced under 

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

 mediate contact with the atmosphere, as in shallow marshes 

 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 

 fellow traveller, 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 sand- 

 stone formations which have also been deposited as sedimentary 

 strata from liquids, and which have been embedded alternately 

 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 thia metamorphisin, and tho 



