702 



SCIENCE 



[N. S. Vol. XLIV. No. 1142 



relations are well displayed, are not obscured 

 by later dynamo-metamorphism, and bave been 

 determined with, notable thoroughness and 

 skill in independent and supplemental work by 

 Crosby, Lougblin and Warren; Professor 

 Crosby's results having been published in 

 1895, Loughlin's in 1911, and Warren's in 

 1913. 



Preliminary explanations of the geology 

 were given with the aid of lantern slides by 

 Professors Crosby and Warren Friday eve- 

 ning, October 27, in the lecture room of the 

 Boston Society of Natural History. The ex- 

 cursion, participated in by 39 persons, repre- 

 senting 12 institutions, left Boston at 8 :49 

 a.m., October 28, and returned at 5 :30 p.m. 

 Superb weather, the beauty of the Blue Hills 

 in autumnal foliage, the geological interest 

 of the rock exposures, and the instructive in- 

 terpretation of them by the leaders, com- 

 bined to make memorable this excursion. 



A synopsis of the geological relations of the 

 Blue Hills complex as they were shown in the 

 course of the day is as follows : 



The invaded sediments are dark, uniform, 

 siliceous argillites of Cambrian age. They 

 were closely folded and were metamorphosed 

 by the contact action of the underlying magma 

 into hornstones. The structural relations show 

 that they are remnants of a cover which be- 

 fore erosion extended over a considerable por- 

 tion of the Quincy granite batholith. Some 

 parts, now marginal, are preserved because they 

 have been down-folded and down-faulted below 

 the present level of erosion. Some isolated 

 remnants indicate by their parallel orienta- 

 tion within the abyssal rocks that they were 

 roof pendants, other outcrops show by their 

 lack of orientation that they were marginal 

 inclusions of greater or less size. These horn- 

 stones preserve within them diabase dikes, 

 showing thus the nature of the advance intru- 

 sions from the magma which gave rise to the 

 Quincy intrusions. The initial age of these 

 dikes was clearly shown by their restriction to 

 the sedimentary cover and one was pointed out 

 which, furthermore, was cut by a thin dike of 

 fine-grained granite. Next in the series to the 

 diabase dikes is the conspicuous rhomben 



porphyry, the matrix of which is as dark as the 

 diabase, but whose composition is in reality 

 intermediate between that of the diabase and 

 that of the granite. The series thus indicates 

 a progressive differentiation, but discontinuous 

 intrusion. The rhomben porphyry is associ- 

 ated with the margins of the sedimentary cover 

 and is found also abundantly as angular 

 blocks, cognate xenoliths, within the next mar- 

 ginal phase. During the process of crystalliza- 

 tion of the rhomben porphyry there was re- 

 peated shattering and invasion by slightly 

 different phases of the same magma. Finally 

 the zone was shattered and invaded by a dis- 

 tinctly differentiated magma, the third of 

 the series. This crystallized as a granite 

 porphyry. It occurs in places in considerable 

 mass, but elsewhere may exist as discontinu- 

 ous fine-grained rims one or two inches thick 

 about the xenolithic blocks of older phases. 

 In places where a cracking but not disruption 

 of the rhomben porphyry occurred there was 

 some thin infiltration of the following magma 

 accompanied by a metasomatic impregnation 

 of the walls to a depth of a quarter to half 

 inch, bringing about by recrystallization an 

 approach to the nature of the later rock. One 

 of the most interesting relations of the granite 

 porphyry was seen in its chilled contacts with 

 an aporhyolite. The latter appears to have 

 formed a thin chilled cover to the batholith, 

 made from the same magma, yet cut by the 

 porphyritic phase. It indicates that the roof 

 of older rocks had here been destroyed after 

 the stage of the rhomben porphyry and that 

 the batholith was in effect partly deroofed in 

 this acidic stage. Following the granite por- 

 phyry came, after another interval permitting 

 some slight further differentiation, the great 

 upwelling of the Quincy granite. This, the 

 main rock of the batholith, was followed later 

 by the feeble injection of a few diabase dikes, 

 closing the cycle. In the opinion of Professor 

 Warren the structure of the roof of the batho- 

 lith indicates invasion chiefly by stoping. 



This small batholith is regarded as a local 

 and structurally high intrusion belonging to 

 the far wider and more complex batholith, 

 which underlies much of New England, which 



