EXOGENOUS OR- SEDIMENTARY ROCKS. 243 



this point of view a plutonic influence may be said to have 

 acted in a certain sense even on the sedimentary strata, 

 especially on the older ones ; but these strata appear to have 

 hardened and to have acquired their schistose structure 

 under great pressure, whereas the rocks which issued from 

 the interior (granite, porphyry, and basalt) solidified by 

 cooling. As the heat of the waters gradually diminished, 

 they could absorb a larger portion of the carbonic acid gas 

 existing in the atmosphere, and were thus fitted for holding 

 a larger quantity of lime in solution. 



The sedimentary rocks, excluding all other exogenous 

 purely mechanical deposits of sand or detritus, are as 

 follow : 



Argillaceous schist of the lower and upper transition 

 series, comprehending the silurian and devonian forma- 

 tions, from the lower silurian strata, once termed cambrian, 

 to the upper strata of the old red sandstone or devonian 

 period, immediately below the mountain limestone. 

 Carboniferous deposits. 



Limestones included in the transition and car- 

 boniferous formations, the zechstein, the muschelkalk, 

 the Jura oolitic limestone, the chalk, and various beds 

 of the tertiary period which cannot be classed amongst 

 the sandstones or conglomerates. 



Travertin, including fresh- water lime-stone, and sili- 

 ceous concretions from hot springs, formations which 

 have not been produced under the pressure of a great 

 body of sea water, but almost at the surface, in shallow 

 marshes and streams. 



Infusorial masses, a geological phenomenon of great 

 importance, as it has revealed to us the influence which 

 organic life has exercised on the formation of a part of 



