302 PROCEEDINGS OF THE AMERICAN ACADEMY. 



facts as presented is, that of " stoping " as advanced by R. A. Daly ^^ 

 and in a somewhat less highly developed form, independently, by 

 J. Barrell.^^ If the Quincy magma has reached its present position 

 by replacement and not by displacement of the country rock, and has 

 not dissolved the invaded rocks, then the writer can see no escape 

 from the so-called stoping method of replacement. 



The first solidified portions of the magma are found in place in sev- 

 eral parts of the field. In eastern and southern Quincy and northern 

 Weymouth we find the fine-granite as the contact phase. Considera- 

 ble portions of this were broken up or invaded by the still fluid magma 

 beneath, as shown by the perfectly sharp igneous contacts of many of 

 the larger masses of fine-granite with the coarse, by the included 

 masses of fine-granite and by the dikes of coarse-granite cutting the 

 fine. This part of the area undoubtedly represents relatively deep 

 levels of the contact (as held by Professor Crosby). In the Pine Hill 

 tract and the Pine Tree Brook reservation the contact-porphyry is a 

 relatively thin zone passing gradually and rapidly into granite and here 

 also are found the basic marginal diiferentiate of the magma, the 

 rhombenporphyry, and also the other phase of lower contact levels, 

 the fine-granite. In both areas there is positive evidence of the 

 movement of the underlying magma, which often resulted in a break- 

 ing up of the earlier consolidated rocks or in invasion of them l)y dikes, 

 or both. In the Blue Hill Reservation proper, where the cover- 

 porphyry attained a great thickness, the transition from the granite- 

 porphyry to the porphyritic phase of the coarse-granite is either very 

 rapid, or as o])served on Rattlesnake Hill, the contact is sharp, indi- 

 cating that there the magma moved under its own cover. Again in 

 and about Slide Notch we find actual dikes of granite (with xenoliths 

 of the dark feldspar-porphyry and fine-granite types) of considerable 

 dimensions cutting the porphyry cover. In several places among the 

 higher hills we have also noted the occurrence of considerable areas 

 of fine, feldspar-quartz-porphyry essentially identical with the contact 

 phases (against the aporhyolite of Wampatuck Hill for example) of 

 the granite-porph\Ty, but in many instances, as for example, on the 

 northern and northeastern slopes of the Great Blue Hill, the contacts 

 of these fine porphyries and the associated granite-porphyry are either 

 very rapid transitions or are really sharp though perfectly sealed 

 contacts, again indicating a movement accompanied by breaking off, 



57 American Journal of Science, No. (1903) 26, (July, 1908). 



58 U. S. Geol. Surv., Prof. Paper, No. 57; (1907). 



