716 A TREATISE ON METAMOEPHISM. 



METAMORPHU' EFFECTS. 



The metamorphic effects accomplished in connection with intrusions 

 are both structural and mineral. It has already been noted (p. 712) that 

 the effects of great masses of intrusive rocks may extend for several or many 

 kilometers. It is by the study of areas adjacent to batholiths that the 

 contact effects are best appreciated. About a great batholith there are 

 commonly various zones of metamorphism which grade into one another. 

 This is true both of the structures which are formed and of the minerals 

 which develop. 



STRUCTURES. 



It has already been indicated that as batholiths are intruded the rocks 

 are deformed by being pushed aside. The deformation is mainly accom- 

 plished by recrystallization, as described on pages 690-696. During 

 the process the majority of rocks take on a slaty or schistose structure. 

 Cleavage developing normal to the pressure is peripheral to the batholith 

 or circumscribes it. The contact effect naturally dies out as the distance 

 increases from the intrusive. Near the intrusive the recrystallized rock may 

 be a coarse-grained gneiss or schist; farther away from the intrusive it may 

 become an ordinary slate; and still farther away a slaty cleavage may 

 not be found unless it had been previously developed in some other way. 

 Between these three zones there are gradations. 



In America peripheral structures in gneisses, schists, and slates are 

 illustrated by the Carboniferous batholiths of New England, described by 

 Emerson;" by the Algonkian batholiths of the region northwest of Lake 

 Superior, described by Lawsou; 1 by the granite batholith of the Black Hills, 

 described by Van Hise; c and by the batholiths of Vancouver, described by 

 Dawson/ 



If a previous regional cleavage exists in an area intruded by a bath- 

 olith, this older cleavage is reenforced if it correspond to the new direction 

 of cleavage, and tends to be obliterated if it be diagonal or perpendicular 



a Emerson, B. K., Porphyritic, and gneissoid granites in Massachusetts: Bull. Geol. Soc. America, 

 vol. 1, 1890, pp. 559-561. 



^Lawson, A. C, The laccolitic sills of the northwest coast of Lake Superior: Bull. Geol. and Nat. 

 Hist. Surv. Minnesota, No. 8, 1893, pt. 2, pp. 24-48. 



c Van Hise, C. R., The pre-Cambrian rocks of the Black Hills: Bull. Geol. Soc. America, vol. 1, 

 1890, pp. 206-212. 



d Dawson, George M. , Report on a geological examination of the northern part of Vancouver Island 

 and adjacent coasts: Rept. Geol. and Nat. Hist. Surv. Canada for 1886, pt. b, 1887, pp. 1-129. 



