PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 517 



Aplyte. Andesyte.] 



In vol. iv, and elsewhere, all the greenstone older than the Ogishke 

 conglomerate is classed in the Lower Keewatin, the oldest (and lowest) portion 

 being considered as igneous. The granite of Saganaga lake cuts a greenstone, but 

 it is not known whether it is the igneous, pre-clastic, portion, or the fragmental 

 portion in which are found in general the jaspilyte masses of northeastern Minnesota. 

 So far as known there is no rock in the state of older date than the igneous portion 

 of the Lower Keewatin. In the case of this amphibolyte it is probable that it is 

 derived from some portion of the Lower Keewatin, a metamorphic rock due to the 

 granitic revolution which centred in the region of the Saganaga lake granite. 

 (Compare Part 1 of this volume.) N. H. w. 



No. 732. APLYTE. 



Found in vein-like forms in No. 731. 

 Kef. Annual Report, x, page 89. 



Meg. A fine-grained, light, pinkish-gray, granitoid rock composed of quartz and 

 feldspar. No section. 



Hi'iiKifks. This rock is from one of the aplyte dikes which cut both the "green- 

 stone" terrane (see remarks under No. 731) and the Saganaga granite. .It is similar 

 to No. 6500, which is from this same locality. u. s. G. 



No. 733. ANDESYTE (?) 



Krojj Rock lake. Exact locality not given, but quite probably from near the portage to West Sea Gull 

 lake (sec. 17, T. 65-5 W.). 



Ref. Annual Report, x, page 89; Bulletin ii, page 119. 



Meg. A fine-grained, very dark, greenish-gray rock cut by veinlets which are 

 composed mainly of epidote. 



Mia. The section shows a fine-grained, somewhat confused aggregate vifeld- 

 *IHU- and green hornblende, with some epidote and chlorite. The feldspar is some- 

 times in elongated, irregularly outlined grains and also in non-elongated grains. In 

 places the elongated forms give the section a resemblance to a diabase in texture. 

 The feldspar is much clouded and kaolinized and fine grains of epidote have 

 developed in it. It has no cleavage, and is very rarely polysynthetically twinned, 

 though there are a few simple twins. The species was not determined, but a test of 

 the rock powder with hydrofiuosilicic acid showed that an abundance of sodium was 

 present, considerable calcium and very little potassium. The hornblende is green 

 and in small plates and fibres. The plates themselves show no well defined crystal 

 outlines, and are usually fibrous around the edges. The rock perhaps represents an _ 

 original andesyte. The minerals, except possibly the feldspar, are secondary, and it 

 may be that this mineral and the whole texture of the rock are secondary also. 



One section examined. 



Age. Archean, Lower Keewatiu or older. u. s. G. 



