12 STRATIFIED PRIMARY ROCKS. [Ch. II. 



former condition of the planet, the different degrees of crys- 

 tallization in the older rocks might have been dependent on 

 the varying state of the menstruum from which they were 

 precipitated. 



The'presence of certain crystalline ingredients in the com- 

 position of many of the primary rocks, rendered it necessary 

 to resort to many arbitrary hypotheses, in order to explain 

 their precipitation from aqueous solution, and for this reason a 

 difference in the condition of the planet, and of the pristine 

 energy of chemical causes, was assumed. A train of specula- 

 tion originally suggested by the observed effects of aqueous 

 agents^ was thus pushed beyond the limits of analogy, and it 

 was not until a different and almost opposite course of induc- 

 tion was pursued, beginning with an examination of volcanic 

 products, that more sound theoretical views were established. 



Granite of igneous origin. As we are merely desirous, in 

 this chapter, of fixing in the reader's mind the leading divisions 

 of the rocks composing the earth's crust, we cannot enter, at 

 present, into a detailed account of these researches, but shall 

 only observe, that a passage was first traced from lava into other 

 more crystalline igneous rocks, and from these again to granite, 

 which last was found to send forth dikes and veins into the con- 

 tiguous strata in a manner strictly analogous to that observed 

 in volcanic rocks, and producing at the point of contact such 

 changes as might be expected to result from the influence of a 

 heated mass cooling down slowly under great pressure from a 

 state of fusion. The want of stratification in granite supplied 

 another point of analogy in confirmation of its igneous origin ; 

 and as some masses were found to send out veins through 

 others, it was evident that there were granites of different ages, 

 and that instead of forming in all cases the oldest part of the 

 earth's crust, as had at first been supposed, the granites were 

 often of comparatively recent origin, sometimes newer than the 

 stratified rocks which covered them. 



Stratified primary rocks. The theory of the origin of the 

 other crystalline rocks was soon modified by these new views 



