85 



At times the typical diabase passes into a rock much 

 coarser in grain, that has been described as gabbro, but 

 many of these coarser varieties, when examined closely, are 

 found to have the ophitic texture. 



DIKES OF APLITE OR GRANOPHYRE YOUNGER THAN N1PISSING DIABASE 



Especially in the Elk lake and Gowganda areas 

 the Nipissing diabase is frequently cut by narrow dikes of 

 aplite or granophyre. The material in these dikes is believed 

 to represent residual, more acid material of the diabase 

 magma. On the cooling of the diabase, cracks were formed 

 in it, and material from the residual magma, rising through 

 the cracks and fissures, formed the dikes of aplite or 

 granophyre. Chemical and microscopical examinations of 

 these dike rocks show that they are genetically connected 

 with the diabase rather than with granite. Compared with 

 the fine-grained granite or felsite dikes in the region, such 

 as those connected with the Lorrain granite, the aplite dikes 

 associated with the Nipissing diabase are found to be char- 

 acteristically high in soda and low in potash, as following 

 analyses show. 



At Cobalt there is a dike of granite on the property of 

 the University mine that cuts the Nipissing diabase and 

 from its chemical composition is seen to be related to the 

 aplites of Gowganda and Elk Lake. Having a much 

 greater width than have the characteristic dikes elsewhere 

 in the region, it is naturally coarser in grain. An analysis 

 of samples from this dike is given below. 



Examined in thin sections under the microscope, the dike 

 rock at the University mine is found to be made up of 

 feldspar, quartz and a colored constituent. The feldspar 

 predominates, and consists of microcline and an acid 

 plagioclase showing fine albite twinning lamellae. The 

 quartz and feldspar occur in allotriomorphic grains, but in 

 two instances show distinct micrographic intergrowths. The 

 colored constituent is not abundant ; it was apparently 

 originally a mica, hut is now represented by chloritic 

 material. 



This dike averages fifty feet in width, while the dikes of 

 the Montreal river area and Gowganda are usually under 

 eighteen inches. 



