B. K. EMERSON — PORPHYRINIC AND GNEISSOID GRANITES. 563 



large feldspars are interspersed. In the Hatfield argillite, a zone of delicate chias- 

 tolites is succeeded inwardly by a zone where the chiastolites are changed to a mixture 

 of muscovite and minute twins of staurolite (the mass still retaining the shape and 

 black cross of the chiastolite) by the influence of greater heat and alkaline solutions ; 

 and nearer the granite the whole changes to sericite schist, chlorite schist, and finally 

 hornblende and feldspar appear near the contact with the hornblende granite. 



The outcrops which have been discussed have for the most part been called granites 

 heretofore. This, however, is true of them, that they arc often entirely indistinguish- 

 able from the Cambrian conglomerate gneiss where both are developed as medium- 

 grained biotite granites. 



The more perplexing cases remain for consideration. These are the broad bands of 

 biotite granite, often well foliated, which stretch across the state with a width of five 

 to twenty-five miles. They may be called the Princeton, Barre, Athol, and Orange 

 bands. The Princeton band starts at the north west corner of Worcester and, gaining 

 soon a width of ten or twelve miles, runs north through Pitchburg, where are great 

 quarries, and on into New Hampshire, where it is called " Concord granite " by Pro- 

 fessor C. H. Hitchcock on the map of the second New Hampshire survey, and classed 

 as " Montalban.'' 



On being mapped, it cuts across the Carboniferous and older schists as an intrusive 

 mass. Though often foliated, it is more often massive, and its foliation cannot be 

 harmonized with that of the adjacent schists. It has a clear zone of contact metamor- 

 phosis — fibrolite schists changes to garnet and staurolite schist, argillite to chiastolite 

 schist, quartzite becomes gneissoid, and tourmaline is developed for miles along the 

 border. 



Lying in the middle of this granitic area, Mount Wachusett is in structure dis- 

 tinctly laccolitic, and owes its existance to a great mass of fibrolite schists — a portion 

 of the former cover of the batholite. If abook be laid on its side with its ends directed 

 north and south and a slight pressure be exerted on the leaves till they bend up 

 slightly and separate, forming three or four lens-shaped cavities, then will the leaves 

 represent the fibrolite schists, and the cavities the intruded granite ; and if now the 

 eastern half or three-fourths be removed, the remainder will be a good model of the 

 mountain. 



I am compelled thus to consider the whole great mass, more than fifty miles lung 

 and above five miles wide, as an elongated batholite occupying a large synclinal in 

 the schists. The Athol band is still more clearly an intrusive block. The other two 

 combine so equably the peculiarities of the Cambrian conglomerate gneisses and the 

 batholitic granites just described that I hesitate as to their interpretation. 



By reason of the pressure for time, the next paper on the programme was 

 read by title only. It was — 



THE PEE-CAMBRLVN ROCKS OF Till'; BLACK HILLS. 

 BY C. B. VAN HISE. 



The paper is printed in full among the memoirs, forming pages 'JO'J-244, 

 with plates 4 and 5, of this volume. 



