B. K. EMERSON — PORPHYRITIC AND GNEISSOID GRANITES. 559 

 The next paper was — 



rORPIIYRITIC AND GNEISSOID GRANITES IN MASSACHUSETTS. 



BY PROFESSOR B. K. EMERSON. 



[Abstract.] 



Referring to an unpublished geological map of the central part of .Massachusetts, 

 and confining attention to the region between the Berkshire limestones and the Boston 

 basin, it was remarked that the country consists of a great series of mica, quartz, and 

 hornblende schists, all presumably Paleozoic, and eight principal bands of highly 

 feldspathic rocks (more or less interrupted), broad where they enter the state on the 

 north and narrowing southward, and for the most part terminating before reaching 

 the south line of the state. They are granites and granitoid gneisses, in small part 

 Archean, in larger part Cambrian, and in largest part intrusive. 



The western band is a complex of Archean and Cambrian — a row of small Archean 

 ovals, exposed by erosion of the Cambrian conglomerates and conglomerate gneisses, 

 extending quite across the state. The Hinsdale area is typical of the Archean ovals. 

 A center of coarse allanite and magnetite gneiss surrounded by a band of coarse 

 limestone, like that of Ticondero^a, carrying phlogopite, chondrodite, etc. Outside 

 the limestone is a graphite gneiss carrying a characteristic blue quartz. 



This Archean series is bounded by a broad area of a coarse Cambrian conglomerate, 

 mostly changed into a white biotite gneiss, like the quarry stone of Monsen and Pel- 

 ham. It is itself quarried extensively in Becket. 



The Allanite gneiss dips beneath the limestone and so outward — the quaquaversal 

 arrangement is perfect, though no special weight is put upon this fact in determining 

 the age of the beds. This is deduced rather from the clearly Laurentian type of the 

 Archean gneisses and limestones, and from the facts (1) that the same conglomerates, 

 in their northward extension in Clarksburg, have been found by Mr. C. D. Walcott 

 to contain Cambrian fossils, and (2) that they rest in strong unconformity upon the 

 Archean series beneath. 



This can be seen clearly in a fine exposure along the brook south of the Dalton Moun- 

 tain Club house, on the old Hinsdale-Dalton road. Archean areas of this type extend 

 across Massachusetts and Connecticut, but to the north are two ovals of different type — 

 the Hoosac tunnel and Clarksburg areas. Here the same Cambrian conglomerates 

 and white biotite gneisses surround areas of a coarse porphyritic granite ; and .Mr. 

 J. E. Wolff, who has developed this difficult territory with the greatest perseverance 

 and success, considers these granites certainly pre-Cambrian, and has proved conclu- 

 sively that the conglomerates are unconformable upon them. 



The broad band that crosses the state east of the Connecticut, containing the North- 

 field and Pelham quarries, agrees with tin' Cambrian conglomerate in character and, 

 I think, in age. It is a broad, very flat anticlinal, throwing oil' the whole schistose 

 series on either flank. It shows traces of pebbles here and there, and contains a great 

 bed of slightly actinolitic quartzite. 



The other bands come under a different category. They lie along large synclinals 

 instead of anticlinals. They are commonly biotite granite — line grained to coarse 

 porphyritic — rarely varying to muscovitic and hornblendic varieties. The texture 



