abstracts: geology 409 



cates that the gabbro had been intruded and had cooled at the period of 

 compression. The varieties of gabbro are probably due tomagmatic 

 segregation. 



The chief varities of Sterling granite gneiss are normal granite occur- 

 ring as a batholith, porphyritic and alaskite phases intruded as sills, and 

 pegmatite dikes. The intrusion of the main masses was accompanied 

 by a thoro granitic injection of the neighboring biotitic schist accom- 

 panied by some pneumatolysis. 



The quartzite includes minor variations containing lime-magnesia 

 . silicates and labradorite. The quartz-biotite schist, with its horn- 

 blendic, calcareous, and feldspathic variations, is regarded as an altered 

 arkosic sandstone. The hornblende schist, chemically similar to gabbro, 

 is believed to be a metamorphosed basaltic tuff. The black pseudo- 

 porphyritic schist has the mineral composition of kinzigite, but differs 

 from the type in its pseudophenocrysts of albite-oligoclase and garnet, 

 and is interpreted as the metamorphic form of a previously altered basalt- 

 ic tuff. 



In the porphyritic gabbro the order of crystallization was (1) feld- 

 spar, (2) diallage. accompanied or closely followed by ilmenite. Pyrite 

 appears to be contemporaneous with magnetite. In the norite the 

 order of crystallization was (1) bronzite, (2) labradorite, (3) diallage, (4) 

 titaniferous magnetite and pyrite. The olivine gabbro crystallized 

 similarly but with olivine in place of bronzite. The pyroxenite merely 

 represents a segregation of the ferromagnesian portion of the gabbro 

 magma and is cut by complementary dikes of plagioclase pegmatite. 

 The gabbro outliers and amphibolite sheets are gradational between 

 porphyritic and quartz-hornblende gabbro. 



The conclusion is reached that the two principal varieties of the gab- 

 bro are due to an incomplete differentiation prior to crystallization but 

 that the minor phases are due to local segregation. 



The Sterling granite gneiss is fairly typical of gneissoid biotite granite 

 in general. Study of the granulation of their feldspar and quartz indi- 

 cates that the normal and porphyritic granites were intruded and cry- 

 stallized during the height of the compression period, the alaskite during 

 the closing stages, and the pegmatite in the alaskite after compression 

 and movement had practically ceased. The time of intrusion is sup- 

 posed to be that of the Appalachian revolution. 



The Lantern Hill quartz in North Stonington is believed to represent 

 replacement of alaskite by quartz during the pneumatolytic stage of 

 granite intrusion. 



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