Dodge.] 894 [February 3, 



first there, and it is so indicated on the geological map in the Atlas 

 of Massachusetts, 1 but the " porphyry," already mentioned, and the 

 other crystallines m Needham and Newton carry the northern boun- 

 dary of this area several miles farther north, if they are to be inclu- 

 ded. In the report of the State Geological Survey, the line of the 

 first band, which I have continued southerly from Weston to include 

 certain crystallines at Rice's Crossing and Grantville, is carried 

 south-westerly across Weston to include conglomerates in Natick. 

 The fact is that, particularly at the angles of the bounding lines of 

 the crystalline areas, the overlying deposits are often so shallow as 

 to be correctly described as patches upon the crystalline areas, rather 

 than as distinct regions to be defined by formal lines. The extensive 

 denudation which has taken place over the whole face of the country 

 has probably carried away with the older rocks, many deposits 

 contained in their depressions, leaving now plane surfaces of the 

 crystallines with no suggestion of what once lay above. The north- 

 south line from Weston to Needham, although traced by actual out- 

 crops, may therefore be inaccurate, and the district to the westward 

 may really be or have been a series of E. N. E. - W. S. W. ridges with 

 more recent rocks between them (see p. 416). If any of these last 

 remain, they will probably be found along a line parallel to, and a 

 mile north of the southern boundary of the town of Weston, or in 

 Natick between a continuation westward of that boundary, and the 

 Boston and Albany Railroad. 



The northern boundary of the band in question forms a series of 

 angles from the north-west in a zigzag course toward the south-east. 

 Hitherto, these angles have been ignored in geological maps. It is 

 particularly difficult to outline the easterly sides of the angles which 

 break the north-west and south-east line, inasmuch as the ranges of 

 the crystallines trend eastwardly, so that in that direction they appear 

 at intervals among the later rocks which cover them and break their 

 continuity at the surface. Thus in following the outline we frequently 

 have to turn back after defining a peninsula shaped area, to pass 

 around a bay of newer rocks which for a short distance occur in suf- 

 ficient quantity to separate the continuous, parallel, crystalline 

 ranges. This northern limit is approximately determined by a line 

 passing half a mile south of Wellesley, and as far north of the High- 

 landville Station of the Woonsocket Division, across Charles River, 

 where this turns from a northerly to a westerly course, and toward 



i Boston, 1871. 



