ERUPTIVE EOCKS. 309 



So far as I know, the English geologists are the only ones who have 

 met the question distinctly and have brought forward instances in nature to 

 prove that igneous eruptions have eaten up practically unlimited amounts 

 of sedimentary rocks. These instances are the so-called granite bosses in 

 Ireland (Mourne Mountains), Scotland, and England (Devon and Cornwall), 

 cutting through upturned Cambrian and Silurian rocks, which are compar- 

 atively undisturbed by the eruption and maintain their normal strike up to 

 these granite masses on either side. 1 



Professor Geikie goes so far (op. cit , p. 550) as to ascribe the vari- 

 ability in composition and structure of intrusive masses to involved and 

 melted-down portions of sedimentary rocks. It would be presumptuous to 

 doubt the correctness of the field observations on which these generaliza- 

 tions are founded, and yet it is not only possible, but has sometimes come 

 under my observation that a granite boss has been found protruding through 

 a given rock or series of rocks, and therefore been judged by the geologist 

 who examined it to be younger than the latter, whereas, in fact, the reverse 

 was the case, and the latter rock had been deposited, or had flowed, around 

 an already-existing granite protrusion. In many cases it is difficult to obtain 

 direct proof whether the protruding or the inclosing rock is the older, and 

 in such a case the probability one way or the other may be dependent on 

 this very question of the capability of igneous rocks to assimilate large 

 masses of sedimentary rocks. 



A case in point is the granite body of Little Cottonwood canon, in the 

 Wasatch Mountains, of which a section some seven miles long has been 

 exposed by the erosion of the canon. The present outcrops of the body 

 occupy an area whose dimensions may be roughly stated as 7 by 15 miles ; 

 and a thickness of some 5 miles of sedimentary rocks abuts against its 

 northern side, the upper members sweeping round and in part covering its 

 eastern portion, and continuing southward in an almost horizontal position. 

 There is no special disturbance of these beds in contact with the granite ; 

 so far as observed, they follow the normal dip and strike induced by the 

 dynamic movement of the region. Neither are there any masses or fragments 

 of sedimentary rocks included in the granite. Regional metamorphism exist* 



'A. Geikie: Text Book of Geology, pp. 541 et seq. London, 188:3. 



