84 



OUTLINES OF GEOLOGY* 



been concerned in their formation, either separately or unit- 

 ed. Thus : from the higher groups of the superior order to 

 the lower strata of the submedial, we find evidence of the 

 continued presence of organized beings; in the marls and 

 clays of the several orders we note deposits from waters in a 

 quiescent state; in the sandstones and conglomerates, of 

 similar deposits from water in motion; in the calcareous 

 formations, -of precipitation from solution, aided by the action 

 of the coralline zoophytes, and mingled with the exuviae of 

 animals. 



In comparing the rocks produced by the action of volca- 

 noes, either active or extinct, with those of the trapp, trachy- 

 tic and porphyritic orders, we cannot hesitate at ascribing 

 the latter to the same general cause of internal heat ; while 

 the transition from the porphyry to the granitoid rocks is so 

 insensible, that we cannot refuse to believe, that the same 

 agency which formed the one, must have been efficient in the 

 creation of the other. Of the unstratified rocks the ophitic 

 group alone falls with difficulty under the general head of 

 igneous origin; but when we consider that carbonate of 

 lime, serpentine, and the magnesian minerals have been fused 

 by artificial means, we need no longer hesitate in ascribing 

 to the same igneous action the rocks of this group, which re- 

 semble, in their mode of occurrence, in dykes, veins, and as 

 the nuclei of mountains, those whose origin is unquestion- 

 able. 



191. Derived from the same general cause, the circum- 

 stances under which the several unstratified groups have 

 taken their place in the solid crust of the earth, are by no 

 means identical. The mode of ejection from craters is that 

 which has alone been instrumental in the generation of the 

 volcanic rocks, whether of the ancient or modern period ; 

 and this seems to be also true of the trachyte. The trapp 

 rocks have not only been ejected from craters, but have 



