Page:The Journal of geology (IA journalofgeology11893univers).pdf/168

for a knowledge of the field relations of the specimens studied. Messrs. Herrick, Clarke and Deming^[1] have also studied a few specimens of the gabbro, both ordinary and orthoclastic varieties, from Duluth, but they have added little to what was already known concerning them, except the suggestion of the possible dependence of the orthoclase-bearing varieties upon their environment for the peculiar characteristics which they possess.

The Canadian geologists have likewise been engaged in a study of the rocks on the north side of Lake Superior. Many allusions have been made to the massive sheets and dykes in the Thunder Bay region, but no microscopical descriptions of them have been published, with the exception of a few notes by the present writer appended to a report by Mr. Ingall^[2] on Mines and Mining in the Thunder Bay Silver District. In this report the relations of the large dykes and thick beds of diabase or gabbro to the fragmental rocks of the Animikie series north of the lake are carefully sketched, and the microscopic features of the most important rocks are described. In the Appendix,^[3] a few altered gabbros and diabases from both sheets and dykes are very briefly characterized. The former of these have the general peculiarities of the gabbro from the great dyke on Pigeon Point, Minnesota, referred to by the writer^[4] in an article on certain contact phenomena at this place, and described at greater length^[5] in a bulletin of the U. S. Geological Survey. In the first of these two papers, in addition to the reference to the Pigeon Point dyke, a few remarks are made concerning the relations of Irving's orthoclase-gabbros to the more common varie-