448 THE JOURNAL OF GEOLOGY. 



If Irving. Williams, and Wadsworth are correct in their opin- 

 ion, 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 ortho- 

 clase 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 cer- 

 tain gabbros from Wisconsin, and Irving 3 discovered it in simi- 

 lar 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 



X R. Pumpelly: Geology of Wisconsin, III, 1880, pp. 38, 40, 41. 



2 A. A. Julien : Microscopical Examination of eleven rocks from Ashland County, 

 Wisconsin. Geol. of Wisconsin, III, 1880, p. 233. 



3R. D. Irving: The Copper-Bearing Rocks of Lake Superior. U. S. Geol. Survey, 

 Monograph V, pp. 50-56. 



4 F. D. Adams: Ueber das Norian oder ober-Laurentian von Canada. Neues. 

 Jahrb. f. Min., etc. B.B. VIII, p. 419. 



s A. C. Lawson : The Anorthosytes of the Minnesota Coast of Lake Superior. 

 Geol. and Nat. Hist. Surv. of Minn. Bull. No. 8, p. 1. 



