

220 STRUCTURAL AND FIELD GEOLOGY 
7. The petrographical character of a batholith is some- 
times considerably affected by that of the rocks it has invaded. 
Apparently this is due to the latter having been to some 
extent absorbed and assimilated by the intrusive mass. 
(6) Regional Metamorphism. — There are extensive 
regions of schistose rocks where plutonic masses are so 
sparingly present that the metamorphism can hardly be 
assigned to their action. When gneiss, mica-schist, etc., are 
found occupying hundreds and even thousands of square 
miles, it is impossible to believe that such broad areas could 
have been affected by the more or less widely separated 
batholiths, sills, and dykes by which they are often traversed, 
Alteration on this extensive scale is known as_ Regional 
Metamorphism. There have been many speculations as to 
its cause or causes. Some geologists, indeed, are inclined to 
the view that regional metamorphism is only contact or 
thermal metamorphism “writ large,” as it were. They hold 
it probable that, although intrusive rocks may appear at the 
surface only here and there throughout an extensive area of 
schistose rocks, nevertheless such an area may be underlaid 
at no great depth by vast plutonic masses. It is impossible 
to deny that this may sometimes or even often be the case. 
There can be no doubt that batholiths which show at the 
surface frequently extend laterally for long distances under- 
ground, and this is one reason for the extreme width 
sometimes attained by the aureole of metamorphic rocks 
surrounding a plutonic mass. It is probable, moreover, that 
the numerous veins and dykes which often crop out at a 
great distance from the visible margin of a granite mass are 
not directly connected with it, but with its underground 
extensions. Nevertheless, when throughout an extensive 
region of schistose rocks no batholiths appear, even in the 
deepest sections, while dykes are either absent or very 
sparingly present—we are not justified in assuming the 
existence of concealed plutonic masses to account for the 
general metamorphism. Cases of this kind call for a different 
explanation. 
(a) Plutonic Metamorphism.—The earliest attempt to explain the 
phenomena in question was made by Hutton—the eminent Scottish 
geologist—who maintained that the crystalline schists were originally 

