THE PRECAMBRIAN ROCKS OF THE CANTON QUADRANGLE 9I 



considerable quantity of metamorphic igneous amphibolite in place 

 at the time of the granite invasion, it is not only reasonable but 

 necessary to suppose that a large amount of this material was 

 inclosed in depth by the invading magma, which later exercised no 

 influence upon the foreign substance further than to soften and. to a 

 limited extent absorb it. Though the larger masses of gabbro-amphi- 

 bolite and granite are not found closely associated with each other at 

 the surface, except in the sigmoid injection zones, it is difficult to 

 conceive the acid magma working its way upwards without encount- 

 ering and inclosing much of this earlier basic rock. Similarly it is 

 impossible, in viewing the problem in its broadest areal aspect, to 

 deny that a large quantity of miscellaneous Grenvilie material must 

 also have been included in the Precambrian granite batholiths, and 

 subjected to complete absorption or, by a diffusion process 

 analogous to that proposed by Adams and Barlow, even to altera- 

 tion to amphibolite. But although this general cause may have 

 been in universal operation throughout these batholithic areas, and 

 may have resulted in the uniform production of amphibolite from 

 various sedimentary rock types, it must be conceded on practical 

 as well as theoretical grounds, so far as concerns the Canton quad- 

 rangle, that preexisting amphibolitic or meta-gabbro-dioritic rocks 

 were a secondary if not correlative or preponderating source of 

 much of the material making up the black inclusions. 



While thus admitting the probability of a twofold derivation of the 

 vast majority of black xenoliths, it is not to be forgotten that some 

 of these bodies may have to be referred to an intrusive origin of 

 apparently later age than the granites. This, however, can hardly 

 be regarded as other than a mere possibility, for the relations here, 

 as with most other isolated masses surrounded by granite gneiss, 

 are somewhat inconclusive. The one occurrence which more than 

 all others prompts a consideration of this interpretation, is a small 

 black oval knoll of rock, about 20 by 30 feet in diameter, about 1.6 

 miles northeast of Pyrites on the northeast side of the road, near 

 the point where this is crossed by the 520-foot contour. Plate 16, 

 lower figure, shows this knoll as viewed from the southeast, and the 

 sketch (figure 20) is its ground plan. There are several similar but 

 smaller masses a short distance west of this point on the opposite 

 side of the road. Contrary to the general habit of hornblende schist 

 inclusions, the pluglike mass is a fine-grained, dense, slightly foliated 

 rock of pronounced igneous aspect and, which is most unusual, 

 weathers in relief. The appearance to the naked eye is much like 



