131 



the indigenous variety, while the latter becomes as gradually 

 merged with the distinctly stratified gneiss ; and hence I am 

 well satisfied that, on the map, much genuine gneiss has been 

 included with the granite. Both the granite and the granitoid 

 gneiss are sometimes porphyritic with feldspar crystals, which 

 are occasionally very large. 



The most interesting area of Montalban granite in Eastern 

 Massachusetts is the long but narrow belt which extends from 

 Worcester to Dracut. • This range has been carefully studied 

 by Mr. L. S. Burbank, and I am indebted to him for the data 

 necessary for its accurate delineation on the map. The boun- 

 daries of this granitic belt have never been drawn with even 

 approximate accuracy on any previous map. The granite 

 appears to be mainly of the indigenous variety, frequently pass- 

 ing into or including distinctly stratified gneiss, especially 

 toward the north-eastern end of the belt, in Ayer and Westford. 

 Yet there can be no doubt that portions of the rock have 

 experienced some extravasation. Such appears to be the case 

 with most of the granite in Worcester and Harvard, and, 

 probably, at many points along the range. From Groton 

 southward to Boylston it is decidedly porphyritic, and in 

 Clinton and Berlin the crystals of orthoclase are very large 

 and perfect, often two inches or more across. Farther south 

 it passes into porphyritic and ordinary gneiss. In Harvard, 

 where the granite adjoins the argillite and conglomerate, the 

 latter show extensive alteration, and the granite contains much 

 slaty material, is destitute of mica, and frequently occurs as a 

 schistose, feldspathic gneiss in which slaty particles take the 

 place of mica, while the feldspar is in imperfect rounded crys- 

 tals, giving a porphyritic aspect to the rock.^ 



The granite in Andover occurs in large masses, apparently 



1 Most of the facts embodied in the descriptions of the rocks in the region of the 

 Nashua Valley, given in this paper, as well as some of the language, have been taken 

 by permission from the sketch of the geology of this valley prepared by Mr. L. S. 

 Burbank, and accompanying my report on the Centennial Geological Map of Massa- 

 chusetts. 



