THE GOWGANDA LAKE DISTRICT, ONTARIO 66i 



length, often showing alignment, indicating motion in the mass during 

 crystallization. 



TABLE I 

 SiOa 50.12 



A1203 15.70 



FcaOs 1.42 Norm. 



FeO 6.89 ?; ^■'^( ^ 



T,ir r\ Ab 22.01 > F=54.82. Sal. = =56.24 



^so 9.50 ^ ^,^^ j 54 



CaO II -30 Nep 1.42 L= 1.42 \ 



Na^O 2.91 Di 23.73 P=23-73) 



K2O 1.07 01 16.22 = 16. 22( 



H,0 + 1.03 Mt 2.09) Fem.=43-34 



II 1.06 (■ ^ \ 



H2O — 0.21 Pyr 0.24 A ! 



TiO. 0.55 111,5,4,3 



S o . 14 Auvergnose 



100 . 84 



I. Normal Diabase, O'Brien Mine, Cobalt. N. L. Bowen, Analyst 



DIKES 



In the Davidson Lake sill, which will be described later at greater 

 length, a well-defined dike of fine-grained diabase was found cutting 

 the normal sill rock. Slender laths of plagioclase averaging o . 3 mm. 

 in length are set in a matrix of augite. The plagioclase is very fresh 

 and ranges from acid labradorite to acid bytownite. Augite fills 

 the interspaces and is sometimes altered to a felt of needles of low 

 birefringence with skeletons of iron ore, like the alteration products 

 described by Pirsson in West Rock diabase.^ Iron ore forms about 

 15 per cent of the rock. There is a little pyrite, but at least some of 

 this has filtered in along tiny seams. No quartz could be found. 



Dikes are exceedingly numerous in the Archaean, often up to 250 

 feet in width and of very uniform character. With the exception of 

 the chilled margin the dikes are in most cases composed entirely of 

 dark-gray, rather fine-grained, normal diabase. Sometimes, however, 

 large phenocrysts of feldspar up to 2 inches in length,'' often with a 

 flow arrangement parallel to the walls, stand out on the weathered 



I Diller, Educational Series of Rock Specimens, 271-72. 



