A study of the diabase dykes of the Canadian shield.

This paper deals with the results of a study and review of the petrographical, stratigraphical, and structural characteristics of certain basic intrusives, generally distributed throughout the Canadian Shield, which are described as "diabase" dykks and sills. It is found that the cutting relationships reported afford positive evidence of at least four sequences of "diabase" intrusion. Except for degree of alteration, the sequences resemble each other so closely that positive age correlation on a petrographical basis is precluded. The dykes have predominant north west, north-south, and north a act trends. The writer concludes thab structural evidence suggests" the hypothesis that the dykes are emplaced in fissures which began opening in the crust above a basaltic substratum, along lines of weakness striking approximately north west, north-south, pnd north east, during stretching of the crust; and that surface trends have been modified somewhat by diversion of the magma into some pre-existing local fissures