PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 403 



Diabase. Aporhyolyte.] 



No. 531. DIABASE (or gabbro, with quartz). 



From the narrow dike-like belt mentioned under No. 530. 

 Ref. Annual Report, x, page 41. 



Meg. Rock of even grain and purplish-gray color, resembling rock No. 526, but 

 of a darker tint. 



^fif. The feldspar is (or was) a plagioclase in crystals of considerable size, but 

 is now so penetrated by pegmatitic quartz and impurities that its optic properties are 

 nearly destroyed. 



There are grains of diallage which yet show the polarization and the character- 

 istic lamination, but the most of the diallage is altered to other substances, or 

 penetrated by other minerals so that it could not be recognized were it not for the 

 visible steps of transition from one extreme to the other. In some cases it seems 

 to form a gray semi-opaque substance resembling letico.reiie; in others it is converted 

 to a green mineral, and in others still it is brown with accumulated hematite. It is 

 not ophitic, but is in small roundish grains which preceded the feldspars. 



Apatite is not uncommon, and biotite is in some conspicuous grains. There was 

 apparently no olivine in the rock, at least nothing can be seen of it in the slides. 



Quartz is not abundant, but forms separate grains in angular spaces between the 

 other grains; also frequently as pegmatitic growths in the plagioclases. Two sections. 



Age. Cabotian. 



Hi'iiiark. The assignment of this rock to an alteration of a basic rock by contact 

 with acid material is necessary not only by force of the field relations, but also of 

 the petrographic characters. There is no independent space which seems to have 

 been occupied originally and wholly by the acid elements, but these are simply 

 insinuated amongst the basic elements after the practical formation of the basic 

 minerals. The transformation must have taken place while the mass and the 

 adjoining rocks were yet hot, and were easily penetrated by liquids and gases. The 

 rock, therefore, is to be considered as a silicified selvedge of a basic intrusive or 

 flow-sheet. This, of course, brings up the question: How much of this red-rock 

 series can be attributed to the same origin? and may not rock No. 526 (the red 

 " granite " of Beaver bay) be assigned to this origin, rather than to the consolidation 

 of an acid magma? In this connection comparison may be made with the descrip- 

 tions of rocks Nos. 263 and 265, Wauswaugoning bay, and with much of the so-called 

 " intermediate rock " of Bayley. (Bulletin cix, U. S. Geol. Survey.) N. H. w. 



No. 532. APORHYOLYTE. (Breccia in diabase. ) 



" Reddish, and sometimes greenish, trap-like rock, surrounding or embracing pieces of the feldspar at 

 Beaver bay. This rock is similar to rock No. 526, at least in some of its parts. The feldspar masses have the 

 appearance of having been carried in this rock, or to have been in situ when it was deposited as a sedimentary 

 rock, and subsequently to have suffered the metamorphosing forces with it." 



Ref. Annual Report, ix, page 33; Annual Report, x, page 41. 



