THE GOWGANDA LAKE DISTRICT, ONTARIO 



663 



Its western edge (the base) has a chilled margin against the sedi- 

 ments. The arkose has been bleached; otherwise there is no notable 

 effect at the contact. As we approach the eastern edge (the top) we 

 find "pink spots" appearing in the diabase. The actual contact 

 could nowhere be found, but at one point the sill rock within a foot 

 of the sediment was seen to consist entirely of a pink feldspathic 



ark 



Fig. I. — Ideal section at Davidson Lake. 

 Archaean granite. 



ark. = arkose; di.= diabase; gr. = 



variety. The microscope shows this rock to be made up of pheno- 

 crysts of acid plagioclase (albite to oligoclase-albite) in a ground- 

 mass of the same material with quartz and a small amount of augite 

 and black iron oxide. 



THE FOOT LAKE SILL 



To the northwest of Foot Lake, diabase is found in contact with a 

 narrow band of Lower Huronian slates {A, Fig. 2), 



W 



Fig. 2. — Ideal section of Foot Lake sill (laccolith), si. =slate; di. = diabase; gr. = 

 granite. 



The slates near the contact normally greenish to pale reddish are 

 changed in color to a deep purplish red. Under the microscope it is 

 found that each lamina of sediment has been transformed to a mosaic 

 of quartz and feldspar with a considerable proportion of chlorite and 

 small grains of black and red iron ore. In the varying proportion of 



