ROCKS OF THE PRE-CAMBRIAN COMPLEX 237 



Hussak 11 in Brazil, and Crosby and Fuller/ 2 Williams, 13 Van Hise, 14 and 

 others in the United States, and J. E. Spurr 15 in Alaska. According 

 to Hussak, the granitic quartz veins in certain instances build narrow 

 contact zones in the slates which they intrude. Such zones would 

 not be very noticeable, except when the veins or dikes are in rocks rich 

 in alumina or lime which readily recrystallize into alumina and lime 

 silicates. 



DIORITE DIKES 



In addition to dikes of the white granite (alaskite), there are numer- 

 ous dark green dikes, usually less than 10 feet in width, but of much 

 greater longitudinal dimensions. These will be referred to in general as 

 diorite dikes, although some of them contain too little feldspar to be 

 properly designated by that name. They are composed of green horn- 

 blende and plagioclase (andesine and labradorite), with accessory apatite, 

 ilmenite, and other minerals, together with secondary products, such as 

 epidote, chlorite, and carbonate of lime. At some points these diorite 

 dikes grade over into hornblendite by loss of feldspar. For ordinary 

 purposes they may be called greenstone. These dikes are evidently later 

 than all the other rocks of the complex, for they cut across the gneissic 

 and schistose structures, and frequently show at the border a fine grained 

 layer or salband, due to the more rapid cooling of the intrusive magma 

 where in contact with the wall rocks. These salbands are to be noted 

 where the dikes are in contact with the white granite and the quartz 

 veins, and hence later than these rocks. The following analyses are of 

 two of the more basic dikes, neither of these being true diorites : 

 Analyses of Greenstone dloritic Dikes from Mineral Ridge 

 George Steiger, Analyst 





Number 208. 



Number 222. 



Silica 



48.67 



8.75 



8.58 



3.39 



.99 



46 28 



Magnesia 



19.54 



Lime 



9 91 



Soda 



2.21 



Potash 



1 89 







11 Hussak : Zeits. fur Praktische Geologie, 1898, p. 356. 



12 Crosby and Fuller : American Geologist, vol. xix, 1897, pp. 156-173. 



13 G. H. Williams : Origin of Maryland pegmatites. Fifteenth Annual Report of the 

 U. S. Geological Survey, p. 679. 



14 C. R. Van Hise : Principles of the North American pre-Cambrian geology. Sixteenth 

 Annual Report of the U. S. Geological Survey, part i, 1896, p. 688. Prof. Van Hise also 

 treats of this subject in his fine monograph on metamorphism. 



15 J. E. Spurr : Geology of the Yukon district, Alaska. Eighteenth Annual Report of 

 the Geological Survey, part iii, 1898, pp. 101-392. 



The notes on granitic quartz veins presented here were enlarged on by J. E. Spurr 

 in Professional Paper of the U. S. Geological Survey, no. 55, and many additional data 

 presented. 



