BROADHEAD — AGE OF OUR PORPHYRIES. 367 



and are closely associated with dioritic, chloritic, and epidotic 

 strata." 



Further on, he says, "stratiform rocks seemingly identical with 

 these quartziferous feldspar porphyries abound in Missouri, 

 where they are associated with the iron ores of Iron Mountain 

 and Shepard Mountain. The breccia and conglomerate, in which 

 is found the native copper of the Calumet and Hecla and the Bos- 

 ton and Albany mines of the Keweenaw peninsula, on the south 

 shore of Lake Superior, is mnde up in large part of the ruins of 

 similar orthophyres." 



Again, in Proceedings of Boston Society of Natural History, 

 April, 1S75, Dr. Hunt mentions the feldspar porphyry, or ortho- 

 phyre. so abundant along the eastern coast of Massachusetts, 

 Maine and New Brunswick, and which passes on the one hand 

 into a jaspery petrosilex, and on the other into a finely granular, 

 almost granitoid, rock. 



In its typical and most common form, it is a fine-grained im- 

 palpable mixture of orthoclase and quartz, generally of red, 

 brown or purple color, and porphyritic chiefly from presence of 

 feldspar (orthoclase) crystals, and often grains of crystalline 

 quartz. This rock is in contact with the fossiliferous Lower 

 Cambrian (Menevian) strata of Braintree, Mass., and is identi- 

 cal with thg porphyries of Southeast Missouri. It is referred to 

 the Huronian rocks. 



Further on, Prof. Hunt says that " the unchanged fossiliferous 

 strata are seen resting on the Huronian rocks, and include in some 

 cases fragments derived from these rocks. These strata are like- 

 wise seen to be older than the Menevian. which at St. John, New 

 Brunswick, includes materials derived therefrom. At Hastings, 

 Ontario, these ancient rocks occupy a position between the Lau- 

 rentian and the fossiliferous beds of the Trenton." 



Prof. T. B. Brooks, in Am. your, of Science for March, 1876, 

 speaking of the rocks, above referred to, near Lake Superior, re- 

 gards them as Huronian. 



Prof. Pumpelly, in Mo. Geological Report for 1872, speaks of 

 Archaean rocks of S.E. Missouri, but does not say whether he 

 considers them as Huronian or Laurentian ; but he infers that the 

 granite of S.E. Missouri may be older than the porphyry. 



