20 Loughlin — Relations between tin Quincy Granite 



irregularities in these boundaries are due to faults of north- 

 south trend, which offset the east-west faults. 



The series includes rather coarse- to medium-grained granite, 

 granite porphyry, and quartz porphyry, all of essentially simi- 

 lar composition. The color of the unoxidized rock is mostly 

 a dart bluish, less commonly greenish, gray; that of the oxi- 

 dized rock pink to red. The chief minerals are microperthite, 

 in which the potassic member is microcline, usually without 

 the characteristic gitter structure, and the sodic member is 

 albite. The latter is equal to, or slightly dominant over, the 

 microcline, and in many thin sections forms outer rims which 

 have the same orientation as the albite streaks within the crys- 

 tal. There is little plagioclase independently crystallized. 

 The femic accessory minerals are riebeckitic amphibole and 

 aegirite in megascopic dense crystals, and also in crystallites 

 enclosed by feldspar and quartz. Aegirite commonly forms a 

 border around riebeckite. The quartz forms glassy grains, 

 more or less darkened by femic inclusions. The only products 

 of deep-seated alteration are inky-blue crocidolite and magne- 

 tite derived from the femic minerals and developed in films 

 along slickensided joint surfaces. Epidote and chlorite 

 are entirely absent, even where the minerals of the rock have 

 been mashed and recrystallized along slickensided fractures. 

 The apparent order of crystallization is : (1) microperthite, (2) 

 femic minerals and quartz. Chemical analyses show but traces 

 of lime and magnesia.* 



Thus the Quincy granite series differs markedly from the 

 biotite-granite series in texture, mineral and chemical composi- 

 tion, and in the absence of alteration products, and is readily 

 distinguished in fresh specimens and especially in thin-sections. 

 There may, however, be some difficulty in distinguishing 

 between highly Aveathered and bleached pebbles of the two 

 rocks. 



The porphyiy phases of the Quincy granite are limited to 

 the Blue Hills range, and mark the upper contact zone of the 

 intrusive mass.f Their present limited extent is believed due 

 to an upward tilting of the mass to the north, before or during 

 the period of folding and compression faulting, followed by 

 prolonged erosion, which has to-day left only the least elevated 

 portion of the inclined upper contact zone. The superior 

 weathering qualities of the porphyry phases in comparison to 

 those of the normal granite account for the relief and position 

 of the Blue Hills. 



* H. S. Washington, Prof. Paper, U. S. Geol. Survey, No. 14, 1903, p. 144. 

 The analysis shows 0"3 per cent CaO, and a trace of MgO. According to T. 

 Nelson Dale (Bull. TJ. S. Geol. Survey, No. 354, 1908, p. 94), one half of the 

 CaO is present as CaC0 3 . 



f For detailed evidence see W. 0. Crosby, op. cit. p. 364. 



