APLITE, QUARTZ-PORPHYRY, AND TONALITK. 331 



Coarse pegmatite dikes are rare in the toualite areas. Tlicir place is 

 taken by aplite dikes, wliioli in many places are very jilxindanr in the 

 tonalite, but are always very narrow. The rock is a tine-iii-nincd ijuartz- 

 feldspar mixture, almost without hornblende or mica. Two miles noi-lli of 

 Leyden Center a road runs west down to Green River. Follow iiit;- this 

 road a half mile west, one reaches a place where it bends shiu-ply south 

 around a projecting s})ur of rock. A dikelike mass which has in part the 

 aspect of an aplite and in part that of a quartz-pc)rpliyry crosses the road at 

 this point. It is 13 rods wide, and stands vertical in the Conwa\- schist 

 and strikes north with it. The dike ends in the blutfs to the south, but 

 can be followed a long wav north. It is a pale-gra\ , rather sniall- 

 porphyritic rock, and is the only rock of this tyj)e in t]\v area. 1 have 

 sometimes thought it an exceptionally massive arkose-gneiss. 



QUARTZ-GABBRO AND QUARTZ-DIORITE, OR TONAI^ITE. 



The syenite of President Hitchcock seems to me to liaAe been origi- 

 nally a quartz-diallage rock, but it is now for the larger part a hornblendic 

 rock; indeed, west of the river the presence of diallage can only rarely l)e 

 rendered certain. The low percentage of silica and the almost complete 

 absence of orthoclase exclude it from the syenites. 



HISTORICAL. 



BASIC SECKETIONS: HITCHCOCIv"S .SUGGESTION OF THE THEORY' OF "SCHLIEREN- 



GANGE." 



In 1819 President Hitchcock mentions "syenite" as the prevalent rock 

 along the Connecticut on the east side — a statement scarceh^ correct — and 

 notes that the proportion of hornblende is rather small and that mica is 

 often present. "Porphyritic syenite is common in this quarter and steatite 

 occurs in its eastern part." The first statement is afterwards retracted, and 

 the second 1 can not explain.' 



In 1823 the same author describes the rock more fully from its two 

 localities, Whately and Belchertown. He notes first the interesting fact 

 that in coming from the westward across Northampton "one passes over 

 the most decided jxranite until he comes within 4 or 5 miles of the village. 



' Geology of the Connecticut River: Am. .lour. Sci., Ist series, Vol. I, 1819, p. 106. 



