232 PROCEEDINGS OF THE AMERICAN ACADEMY. 



relatively deep projections into the invading magma, the width of the 

 fine-granite zone is measured by a few feet. Where it marks the 

 more elevated main contacts, its thickness is considerable. While 

 it is not possible to estimate accurately what this thickness was in any 

 case it was doubtless measured by tens of feet. Its contacts with the 

 coarse-granite are exposed at several points and are usually, if not 

 always, perfectly sealed but sharp in character. Professor Crosby 

 believed ^^ that these sharp contacts were due to differential movement 

 of the still unconsolidated magma against its solidified margin, which 

 was generally fractured and invaded by the granite magma as an 

 incident to the process of intrusion. The facts in the field appear to 

 support this reasonable hypothesis for there are numerous dikes of the 

 coarse in the fine-granite and also inclusions of the fine in the coarse 

 (see Crosby, op. cit., p. 352 et seq.). Professor Crosby has noted the 

 occurrence of at least one contact where there appeared to be some 

 gradation in texture, but in the writer's experience it can be said that 

 the gradation is at most confined to a few inches (compare statements 

 regarding the porphyry-granite contacts beyond). 



It is to be noted especially that there are no segregations in the 

 normal fine-granite, a point that will be taken up later in discussing 

 the relations of the various phases. 



The general distribution of the fine-granite is shown on the map. 

 For a more detailed mapping of this rock and its relations to the slate 

 and coarse-granite the special maps of Professor Crosby should be 

 consulted. ^° 



Megascopic. — This rock is a fine grained (one to two millimeters) 

 slightly porphyritic one, of a prevailing light gray color though often 

 light brown or pinkish from alteration. The phenocrysts are alkali- 

 feldspar, are rather sparsely distributed and of characteristically 

 elongated, rectangular outlines (1 by 3 mm. to 2 by 7 mm.). They 

 are more abundant and conspicuous in some localities and become 

 somewhat larger and more prominent in varieties that mark a proba- 

 ble gradation toward the granite-porphyry and probably also toward 

 the slate contacts. Rarely rounded quartz phenocrysts occur, but 

 these are wanting in the normal rock. Scattered quite plentifully 



29 op. cit., p. 355. 



30 It should be noted in this connection that a part of Crosbj-'s fine-granite 

 in northern Weymouth and Hingham is an entirely distinct rock from the fine- 

 granite here considered. It belongs to the biotite, microcline-plagioclase 

 granite which occupies extensive areas to the south and southeast of the 

 alkaline rocks. 



