SUMMARY 475 



New England, and their age is usually considered as Triassic. There is 

 probably another older series of similar dikes, judging by the extensive 

 alteration of some of them. In the Blue Hills, near Quincy, Massachu- 

 setts, there are diabase dikes which cut Middle Cambrian slates, but which 

 are older than the pre-Carboniferous riebeckite granite. 



Several diabase dikes occur in the vicinity of Iron Mine Hill cutting 

 the pre-Cambrian schists and the gabbro. In the northeastern corner of 

 the area, south of Uncas Pond, is a dike cutting Joes Rock quartz por- 

 phyry. Boulders have been found in whii^h diabase cut riebeckite gran- 

 ite ; therefore some of the dikes must be younger than those of the Blue 

 Hills, and it seems best to refer them to the Triassic. 



Summary 



The geological history of this region as now exposed begins in pre- 

 Cambrian times with the deposition of what is now the Cumberland 

 quartzite. With this sediment were deposited beds of shale until finally 

 the sedimentation was entirely of shale, with some conglomerate com- 

 posed largely of pebbles from the earlier sandstone. A small amount of 

 limestone is interbedded with the shale. A part of the schist is probably 

 of igneous or" gin. After the close of the sedimentation, masses of gabbro 

 and probably the cumberlandite, with accompanying dikes of labradorite 

 porphyry, were intruded into the sediments. 



A period of diastrophism followed the pre-Cambrian sedimentation and 

 initiated Lower Cambrian sedimentation, only two remnants of which 

 are now exposed — at Hoppin Hill and south of Joes Rock. 



During the Acadian revolution of Middle Devonian age the biotite 

 granites were intruded, accompanied by quartz diorites and fine granites 

 or fine granite porphyry as facies of the main intrusion. At the begin- 

 ning of Pennsylvanian time the deposition of the Narragansett and Bel- 

 lingham series of shales, conglomerates, and sandstones with interbedded 

 volcanics commenced. In Middle Pennsylvanian time a period of dias- 

 trophism interrupted the sedimentation. The riebeckite granites were 

 intruded, followed by the injection of quartz veins at Diamond Hill. 

 Sedimentation recommenced with the deposition of the Dighton con- 

 glomerate of the Narragansett Basin, but was brought to a close by the 

 Appalachian revolution. Since this time the region has been one of ero- 

 sion. A few diabase dikes were intruded probably in Triassic time. 



The various sedimentary and metamorphic-sedimentary rocks have been 

 briefly described. A summary description has also been given of the 

 gabbro and the ultra basic rock, cumberlandite. A megascopic and micro- 



XXXIV— Bull. Geol. Soc. Am., Vol. 25, 1913 



