Page:The Journal of geology (IA journalofgeology11893univers).pdf/22

If Irving, Williams, and Wadsworth are correct in their opinion, the hornblende-gabbro of Streng is merely an altered form of gabbro, and therefore it does not deserve a distinctive name (except for the mere purpose of description), any more than do the saussurite-gabbros.

Another type of gabbro to which a distinctive name has been given is also found in the region surrounding Lake Superior. This is an orthoclase-gabbro which has been carefully described by Professor Irving. An unstriated feldspar taken to be orthoclase had been discovered in gabbros from European localities by various petrographers, but it was usually present in such small quantity that but little importance was attached to it. In this country Pumpelly^[1] and Julien^[2] identified orthoclase in certain gabbros from Wisconsin, and Irving^[3] discovered it in similar rocks from both Wisconsin and Minnesota. The latter author describes the orthoclase as often reddened and charged with secondary quartz. He mentions in detail the characteristics of the rocks containing it, and regards the differences noted between these and the non-orthoclastic gabbros as of sufficient importance to warrant their separation from the latter under the variety name orthoclase-gabbro.

Within the past few months still an additional gabbro variety has been brought into prominence by Adams^[4] and by Lawson^[5] working in different portions of North America. This consists essentially of plagioclase with gabbro characteristics, with which is associated only now and then a grain of pyroxene or magnetite. It differs from "forellenstein" in containing no olivine, and from