82 EDWARD STEIDTMANN 



grading into a mosaic of hornblende and feldspar crystals. With 

 diminishing hornblende the gradation passes into a pyroxene, 

 scapolite, biotite rock, and finally into coarse crystalline limestone. 



From analyses of limestones representing three stages of altera- 

 tion to amphibohte near a contact, Adams concludes that the 

 alteration has involved the introduction from the granite of silica, 

 alumina, iron, magnesia, and alkalies, and the loss of lime and 

 carbonic acid. This conclusion seems to rest on the assumption 

 that the alteration took place without change of mass. 



Lindgren^ presents a resume on the principal epochs of the 

 segregation of metals on the North American continent. Many 

 different epochs of ore formation are embraced in the pre-Cambrian 

 period (in many different places) . In the eastern part of the con- 

 tinent iron, copper, nickel, silver, and gold ores occur in association 

 with a variety of both intrusive and extrusive igneous rocks ranging 

 from granite to basalt. In the Cordilleran region, intrusives, 

 principally granites, are almost exclusively represented. Intrusive 

 diorite, gabbro, and diabase are present. The Cordilleran pre- 

 Cambrian ores are principally gold and silver. The lead and zinc 

 ores of the pre-Cambrian are unimportant. 



Van Hise^ maintains that the bases of classification of the pre- 

 Cambrian should be physical, those of most importance being: (i) 

 lithologic character, (2) continuity of formations, (3) hkeness of 

 formations, (4) like sequence of formations, (5) subaerial or sub- 

 aqueous deposits, (6) unconformities, (7) relations to series of 

 known age, (8) relations with intrusive rocks, (9) amount of 

 deformation, (10) degree of metamorphism. His major divisions of 

 the pre-Cambrian are Archaean and Algonkian. 



1910 



Adams^ concludes that large igneous intrusives rising from the 

 deeper portions of the earth are one of the principal agents of the 



' Waldemar Lindgren, " Metallogenetic Epochs," Econ. GeoL, IV, No. 5 (1909), 

 409-420. 



^ C. R. Van Hise, "Principles of Classification and Correlation of the pre-Cambrian 

 Rocks," Jour. GeoL, XVII, No. 2 (1909), 97-104. 



3 Frank D. Adams, "The Origin of the Deep-seated Metamorphism of the pre- 

 Cambrian Crystalline Schists," Compte Rendu, Congres Geologique International, 

 1910, pp. 563-72. 



