151 F. R. Lahee — Metamdrphism and Geological Structure. 



contrasts the more intense thermal effects occasioned by basic 

 intrusives, which contain few or no mineralizers, with the less 

 intense effects of granites and other acid rocks.* He regards 

 pneumatolytic metamorphism, however, as limited to a rather 

 narrow zone next the contact. f 



It is probable that the temperature of the magma at the 

 time of its injection is not excessively high. This is indicated 

 in the country rock by the absence of proof of true fusion 

 along the contacts and by the abundance of minerals which 

 developed during the metamorphism and which can crystal- 

 lize only at relatively low temperatures — considerably below 

 their freezing-points. Such are the micas, the amphiboles, the 

 alkali-feldspars, quartz, tourmaline, and many others. J Since 

 these minerals are also among the more important constituents 

 of the granite itself, they prove a comparatively low tempera- 

 ture of consolidation for the magma.§ 



The ragged edges of inclusions (xenoliths), and, adjacent to 

 the country rock, the coarseness of texture of the intrusions, 

 do not of necessity prove a very high temperature for the 

 latter. Magmatic solution (not fusion)! may account for this 

 feature in the inclusions, and moderate warmth will explain 

 the coarse texture. 



The granite phase: Description. — Coming now to the 

 description of the Acid Intrusive Series, we find that the 

 Boston Neck granite occurs at various places along the western 

 coast of Narragansett Bay from Watson's Pier (between Iocs. 

 11 and 12, C : 15, tig. 1) southward, and inland on Little Neck 

 (B : 16, ftg. 1), Boston Neck, and Tower Hill (A and B : 14 and 



15). . . . 



This rock is typically of medium grain ; is white, pinkish, or 

 cream-colored ; and consists predominantly of microcline, with 

 quartz, micropegmatite, microperthite, a coarsely twinned 

 plagioclase 7 and orthoclase. A little biotite or muscovite may 

 be present, and apatite, zircon, magnetite, rutile, and garnet, 

 are among the less common constituents. Megascopically, a 

 varying degree of schistosity is seen. Under the microscope, 

 the quartz appears somewhat strained, the feldspar grains may 

 be cracked or faulted, and the micas are more or less bent. 



* Harker, A., op. cit., p. 189. 



f Ibid., p. 304. 



tlbid., pp. 187, 284. 



§ Harker (op. cit., p. 186) quotes Lehmann's estimate of 500° as "probably 

 by no means too low. " "Referring to Sorby's investigations on the vacuoles 

 of crystals, he shows that the results obtained by Sorby lead to the same 

 conclusion (pp. 187-189). See also Geikie, A., op. cit., pp. 412-413. E. S. 

 Bastin (op. cit., pp. 38-39), applying the data of Wright and Larsen, arrives 

 at the conclusion that pegmatites crystallize not far below, nor far above, 

 575°C. (See Wright, E. F., and Larsen, E. S., Quartz as a Geologic Ther- 

 mometer, this Journal, iv, 28. 1909.) 



I Geikie, A., op. cit., pp. 767, 776. 



