PETROGRAPHIC GEOLOGY AND DESCRIPTIONS. 893 



Quartz-porphyry.] 



angle of about 30. Other feldspars are distinctly twinned on the albite plan. 

 Otherwise this rock is much like No. 407G. One section. 



Age. Archean. N. H. w. 



NO. 410G. QUARTZ-PORPHYRY. 



South shore of small lake, W. y z N. E. y sec. 21, T. G3-10 W. From a dike, within two inches of its edge, 

 in greenstone. 



Ref. Annual Report, xx, pages 56, 57, 100. 



Meg. A very fine-grained, pinkish rock, with porphyritic quartzes and abundant 

 pinkish porphyritic feldspars. 



Mic. Similar to No. 417G, except that the groundmass is some finer grained 

 and the feldspars are not usually in well defined crystals. One section. 



Aye. Archean. u. s. G. 



No. 417G. QUARTZ-PORPHYRY. 



Prom t'ae centre of the same dike (see under No. 416G). 



Ref. Annual Report, xx, pages 56, 57, 100 ; Annual Report, xxi, page 43. 



Meg. Similar to No. 416G, but more granite-like in appearance. 



M'n-, Microscopically the rock shows a decided microgranitic groundmass of 

 rather small but irregular grain. Embedded in this are numerous feldspar pheno- 

 crysts of all sizes up to those nearly half an inch in length. Most of these feldspars 

 show their crystallographic outlines on all sides, but some few appear as fragments 

 partially bounded by crystal planes. Zonal structure is quite common, and about 

 half of the individuals show polysynthetic twinning lamellae. Quartz individuals 

 of good size are also present, but are not nearly as abundant as the feldspars. The 

 quartzes are all corroded and show no crystal faces, and they frequently have large 

 embayments filled in with the groundmass. Some of the quartz shows undulatory 

 extinction, but otherwise the rock gives no evidence of having been subject to 

 pressure. Scattered through the groundmass are irregularly outlined green areas 

 composed of chlorite and epidote. What the original ferromagnesian constituent of 



the rock was is now impossible to determine. A' few small acute rhombs of sphene 





 are also present. Two sections. 



Chemical analysis. An analysis of this rock, made by Mr. A. D. Meeds, is as follows: 



Si0 2 69.70 



A 1,O, 18.72 



Fe,O, .65 



PeO .79 



CaO 2.25 



MgO .45 



K 2 O 1.68 



Na 2 O 5.01 



H a O .71 



Total 99.96 



Aye. Archean. u. s. G. 



