434 



THE AZOIC SYSTEM AND ITS SUHDIYISIONS. 



that the Iluronian granites of this region are inaiulj exotic. Wo have seldom 

 far to look to fiud, iu the form of ench)sedj ari^ndar fragments of cleai'ly-strati- 

 lied rocks, evidence of their extravasation ; and near tlie Loundaries of the 

 granites we often observe them cutting the adjoining rocks, especially if tliese 

 are stratifiedj in a manner incompatible with any theory that would rcf-ai'd 



them, hi their })resent conditioii, as chiefly indigenous Believiu<'' with 



Prof. Shaler, and also Avith Mr. T. T. IJouve, that all these granites are meta- 

 morpliosed sediments, I conceive that the peculiar planes of separation (joint 

 structure) relV.ri'ed 1o by I'rof. Shaler demand a dilferent interpretation from 

 that proposed by him, for evidence is not wanting of the extravasation of the 

 granite at many points along the Blue Hill or Quincy and Milton range. 

 . . . . It is well known that the Quincy granite is met along its northei'n bor- 

 der l)y conglomerate and slate The actual contact of this rock with the 



granite is displayed, however, at a i)]ace about J mile west of the Old Col- 

 ony K. R The contact Une is extremely iiTcgular ; and tlie relation 



of the granite to the semi-crystalline rock is un(|uestionably that of an exotic. 

 Staae three miles to the southwest, it is very distinctly cut by dykes and irrt'*^- 

 ular strings of tlie underlying and surrounding gi'anite. According to I'rof. 

 W. n. Niles, the relations of the granite and slate on Weymouth Fore River, 

 near the trilobite fj_uarry, affords equally conclusive evidence that at least a 

 portion of the granite has ex])erienced some extravasation since the dejiosition 

 of the slate. The slates on the South Sliore 11. R,., innnediately east of the 

 station at Weymoutli Landing, are in contact with the gi'anite, which cuts 

 througli, and overlies them iji a manner possible, ap])arently oidy \\-i!h an 

 exotic ; and at the contact of the granite and slate, southwest of the station, 

 l^rof. Niles has observed angidar fragments of slate actually enclosed in the 

 granite, though lying only a few inches from their original positions in tlie 

 parent hvA. The induration, as if by heat, of the slate and conglomerate at 

 most })oinls where they adjoin the granite, and the frequent devehjjinu^nt of 

 amygdaloidal characters in the slate in those places, are also facts which tell 

 strongly in favor of the foimer igneons condition of the granite. The evi- 

 dence of the extravasation of the granitt; afforded by a study of its relations to 

 the uncrystalline rocks ap])ears to be sulliciently conclusive as regards the por- 

 tions of granite innnediately involved At Hospital Point on the Bev- 



erl}^ sliore, near the water's edge south of the lighthouse, is a consideral)le mass 

 of distinct mica-shite enclosed in the coarse, structureless granite. A smaller 

 mass of a similar slr;LtiHed schist is enclosed in the granite near tlie mtrthern 

 end ot the railroad-cut in Beverly. On Marblchead Neck the relations of tlie 

 granite to the ilne-grained, distinctly stratified schist occurring there, are sucli 

 as to leave no doubt that the granite is exotic. Along the siioi'o, at the south- 

 western end of the nock, the exposures? are inagiiificeut, and one can see, espe- 

 cially at low^ tide, numerous angular, ragged, contorted masses of the schist, of 



various sizes, enveloped by the gi';tnite. .... It has been set down by all 



observers as the oldest roidv in this region, and this view is abundantlv iusti- 



~ ' II.'- 



fied (except as regards the Xaugus Plead series), not only by its generally 



