436 E. S. MOORE 



replacing the feldspar. This arrangement was seen where the path 

 of an epidote crystal was cut across by that of the growing, zonally 

 built feldspar. In a couple of cases a group of epidote crystals is 

 crossed by feldspar, but the simultaneous extinction of all parts of 

 the epidotes shows them to be parts of the same crystal and in one 

 case particularly a portion of the epidote crystal has passed through 

 the feldspar, forming one of the zones of the crystal. In these cases 

 the epidote is undoubtedly primary, although considerable second- 

 ary epidote occurs from alteration of the feldspars. 



An interesting example of a crushed feldspar is seen in this 

 section where the crystal has been broken into slivers and the 

 fragments surrounded by quartz. This must have been due to 

 pressure, although the rock as a whole does not show evidence 

 of excessive pressure beyond the undulatory extinction of some of 

 the quartz grains. 



In his description of the copper-bearing rocks of Lake Superior, 

 Irving describes some sections from dikes of red rock in the Duluth 

 gabbro which would indicate that they are probably similar to 

 those dikes described above. 1 



These acid dikes appear to be differentiation phases of the 

 Keweenawan diabases and gabbros because they occur, with one 

 exception, in these rocks only, and, so far as observed, only in the 

 larger masses and not in the thin sills which are too small to pro- 

 duce them by differentiation. This one exception is a dike 30 

 inches wide cutting quartzite and the overlying diabase near 

 Ombabika Narrows. 2 This dike might be due to the rising of the 

 liquid from some large diabase mass below through a fissure extend- 

 ing into the overlying rocks. There are no other bodies of acid 

 rock in the region later in age than the Keweenawan diabases, and 

 the pegmatitic and micropegmatitic textures suggest end phases 

 of a magma. These dikes probably fill crevices in the diabase 

 formed in the solidified exterior of a large mass, due to adjustment 

 of pressures during processes of cooling, and the acid material 

 rose from the still hot and more acid lower portions of the mass. 

 The lact that these dikes are so much more basic on the whole than 



1 U.S. Geol Survey, Monograph V, 119-20. 



2 Coleman, Bureau of Mines of Ontario, XVII, 164. 



