80 Loughlin — Relations between the Quinoy Gra/nite 



tlu- sediments, though epidote, chlorite, and salcite are con- 

 spicuous in places. Therigid biotite-granite batholith yielded 

 to compression, not through folding, but through complex 

 fracturing, and in these fractures, as well as throughout the 

 rock, the secondary minerals, quartz, sericite, epidote, chlorite, 

 and calcite, were formed through alteration of lime- and mag- 

 nesia-alumina silicates. The slickensided character of the frac- 

 ture fillings shows that these minerals had, in part at least, 

 been deposited before the movements ceased. 



The Quincy granite, containing hardly more than a trace of 

 either lime or magnesia, was chemically more stable under the 

 conditions of compression. Even the highly fractured exposure 

 at the fault contact in Blue Hill village shows no conspicuous 

 secondary minerals, other than quartz in veinlets, which may 

 well have been derived from the contacting arkose. Crocido- 

 lite and magnetite (as. alteration products) appear limited to 

 films along slickensided joints. 



The greater freedom of the Quincy granite from minor com- 

 plex fractures would seem dependent on this same condition. 

 Its greater immunity from alteration gave it greater resistance 

 to the compressive movement, and it yielded by relatively few 

 persistent fractures, which are represented in its quarries to-day 

 by highly slickensided joints of nearly east-west strike and 

 either northward or southward dip. The greater tendency of 

 the biotite granite to alter weakened its resisting powers, and 

 it yielded, not only along major joints and faults, but espec- 

 ially along the numberless minor fractures, in which the 

 secondary minerals had the opportunity to form. 



Subsequent events in the geologic history of the district, 

 while of general interest, are of no great immediate importance. 

 The only events which deserve mention here are : (1) develop- 

 ment of the system of faults, with nearly north-south trend, 

 which offset the compression faults; (2) long continued erosion 

 which resulted in the dissected Cretaceous and Tertiary* pene- 

 plains and removed the sediments from the upthrow sides of 

 the compression faults ; and (3) glaclation, which has greatly 

 obscured field evidence. 



Further Questions of Correlation. — The writer showed in a 

 former paperf that sediments generally l'ecognized as Carbo- 

 niferous along the western edge of the Narragansett JBasin 

 were older than the Sterling granite batholith — a reddish 

 gneissoid biotite granite without marked hydro-thermal altera- 

 tion, and with a minor phase of muscovite granite. It is 

 shown in the present paper that the sediments of the Boston 

 and Norfolk Basins, also generally recognized as Carbonifer- 



* See W. O. Crosby, op. cit., p. 538. 



f This Journal, vol. xxix, pp. 452-456, May, 1910. 



