AND ANALOGIES, OF ROCKS. 



have an exact counterpart of that alternation so com- 

 mon between the oldest of the secondary sandstones 

 and its concomitant shale. I need not dwell longer on 

 this question as far as regards detail; but the whole 

 admits of one general conclusion, considerably at 

 variance with the popular opinions of geologists and 

 systematic writers respecting the differences of rocks. 

 It has been always said that these differences were 

 inexplicable, and that we could not account for the 

 great variety of rocks in nature, still less for that con- 

 stancy which they retain under their variations, nor 

 for the alternations of the different kinds. Now I have 

 shown, that as far as their varieties are concerned, we 

 have, with magnesia, but three materials,, lime, clay, and 

 quartz, and that these, either simply, or in combina- 

 tions which are chiefly binary and between the two 

 last, form the leading materials of all the rocks. In 

 the purely mechanical, or in the aqueous rocks, the 

 variations could be little more than variations of pro- 

 portion, in those formed of more than one material ; 

 and since the general law of deposition and conso- 

 lidation has been nearly uniform, a corresponding 

 uniformity of result is a necessary consequence. In 

 the more properly chemical rocks, or in those which 

 have participated in the effects of heat, a simple law 

 also, varying merely in intensity, and always acting 

 on the same limited materials, differing merely in their 

 relative proportions, has almost necessarily produced 

 corresponding or analogous results. Taking granite 

 thus as the extreme case, the effect of this agent has 

 been to recompound the two leading materials which 

 it found, namely silex, or quartz, and clay, into felspar, 

 mica, and hornblende, of which two, at least, form 

 also the essential parts of the other extreme case, trap; 

 the cause of difference here also consisting in other well- 



