104 



times decidedly coarse, and Mr. Very has observed the same 

 in Foxborough. It is quite possible, as already suggested, 

 that these coarse varieties should be referred to the Nauffus 

 Head series. This area is probably continuous, under the band 

 of uncrystalline strata to the westward, with the smaller area 

 along the New York and New England R.R. The characters 

 of the rocks, so far as observed, are substantially the same.^ In 

 Medfield, Dover, and Norwood, also, the diorite is mainly fine- 

 grained, frequently compact, approaching felsite. These south- 

 ern dioritic areas do not embrace so large a proportion of the 

 quartzose diorite and fine-grained, hornblendic granite as those 

 to the north and east. 



Of the distribution of the diorites little need be said. It is 

 very obvious that they obey the same general law as the gran- 

 ite and petrosilex, forming ranges or belts trending approxi- 

 mately north-east and south-west. In the northern half of 

 Essex County, the diorite is found mainly to the west of the 

 granite and petrosilex, and the difierent belts are united in that 

 direction, cutting off and enclosing the belts of granite. Far- 

 ther south we have a broad and nearly continuous zone of diorite 

 reaching from Natick to Essex, parallel with and mainly to the 

 north of the Natick and Rockport line of granite. Wherever 

 occurring, the diorite is adjacent to granite or petrosilex. In 

 its normal position it overlies the petrosilex ; and hence its 

 contacts with granite must be due to faults, to the extravasation 

 of the granite, or to the conversion of the petrosilex at these 

 points into granite. The diorites of Salem, Swampscott, and 

 Marblehead appear to lie outside of the Natick and Essex 

 range, and are nearly separated from it by granite and Naugus 

 Head rocks. . The diorites, like the granites, are divided by the 

 Naugus Head area, passing to the north and south of it. 



1 Kecent observations have shown that in the vicinity of Franklin Centre and City 

 Mills the rock is not principally diorite, as represented on the map, but coarse, typical 

 granite, which appears to cover a wide area in the region north of the railroad, marked 

 as general Huronian; while the correlation of the coarsely crystalline basic rocks north- 

 east of Wadsworth's Station, with the Naugus Head series, is regarded with even 

 greater satisfaction than heretofore. 



