18 TRANSACTIONS OF THE [OCT. 19, 



weifjht with the writers. The other is that it represents only the 

 upper portion of a dike, and thus contains the float-material which 

 the advance of an i^-neous body would naturally gather from the 

 walls. The lack of such inclusions in the neighboring" dikes may be 

 due to the fact that their tops have been eroded. A still different 

 view has been advanced by Pves. Hitchcock as applicable to a second 

 similar dike, to which we now pass. 



This second dike is much narrower, and is found on Nash's Point, 

 some miles south. It is about twelve feet wide, and consists of 

 somewhat rounded fragments of Archaean norite, red Cambrian 

 (piartzite, and Hudson River slate, chiefly gathered in the middle 

 portion, and all cemented together by the bostonite ground-mass, 

 whose analysis was given above. President Hitchcock^ mentions, 

 also, gneiss, hornblende-schist with garnets, quartz, gray sand- 

 stone, and black Trenton limestone. Probably what he called 

 granite we have called norite. The slides of norite and quartzite 

 exhibit dynamical effects of a marked character, as the crystals have 

 been strained and shattered. These inclusions indicate a deep- 

 seated source for the igneous rock — at least beneath the norites and 

 gneisses, and one or two inclusions in a basic dike from Burlington 

 indicate the same for the basic dikes. President Hitchcock was 

 impressed by the rounded, or, as he interpreted it, the waterworn 

 character of the inclusions. He was led by this to regard the dike 

 as originating from a crack which had become previously filled with 

 deliris from above. This deln'is he considers to have been partially 

 melted or sintered in the metamorphism of the region. As some of 

 the boulders are limestone, and as there are many undoubtedly intru- 

 sive igneous dikes within a few feet, we regard the rounded character 

 as due to partial absorption, and consider the foreign bodies as inclu- 

 sions. Brecciated porphyries formed of a broken eruptive rock that 

 has been re-cemented have been mentioned by Pumpelly from Pilot 

 Knob'^ and by Crosby^ from eastern Massachusetts, but the phe- 

 nomena are obviously different from those here described. 



The Basic Dikes. 



The basic dikes are all compact, dark rocks, that to ordinary 

 microscopic examination, give almost no indication of their consti- 

 tution. The only apparent minerals are feldspars in the coarser 

 diabases, and an occasional glistening hornblende or augite prism 

 in the more basic dikes. The thin sections exhibit a series of 

 niineralogical mixtures extending from very typical ophitie diabase 

 through camptnnites, in which the dark silicates become idiomorphic, 

 and the feldspars recede, to aggregates of augite and hornblende, 



1 E. Hitchcock, On Certain Conglomerated and Brecciated Trachytic Dikes 

 in Vermont, etc., Proc. A. A. A. S., xiv, 156 ; Geol. of Vermont, vol. ii, p. 583. 



2 R. Pumpelly, Geological Survey of Missouri, Preliminary Report on the 

 Iron Ores and Coal Fields. E. llaworth, Missouri Geological Survey, 1891, 

 Rull. V. 



3 W. 0. Crosby, Geology of Eastern Massachusetts, p. 50. 



