THE PROBLEM OF THE ANORTHOSITES 233 



bands he says: "The rock is on the whole richer in bisilicates than 

 the Morin anorthosite, approaching more nearly a normal gabbro 

 or norite in composition." 1 Practically the same statement is 

 made of the Cathcart bands; 2 and again of the Brandon bands 

 he says: ''Like most of the small anorthosite bands described in 

 this report, these from the township of Brandon are usually richer 

 in bisilicates than a true anorthosite should be." 3 



Apparently, then, these outlying bands always vary from typical 

 anorthosite, usually toward gabbro, but in one or two instances 

 perhaps the variation is toward syenite-granite, as suggested by a 

 content of hornblende and biotite. The bands are by no means 

 inconsiderable bodies, usually having a width of upward of half a 

 mile or more and a length of several miles. Even masses of this 

 size are apparently never made up of nearly pure plagioclase rock, 

 a fact that accords with the belief that a fair proportion of other 

 minerals is necessary before anorthosite acquires appreciable 

 invading power in masses of limited size. 4 



CONSIDERATION OF ANORTHOSITES IN GENERAL 



The agreement of the two most completely described areas of 

 anorthosite on the North American continent with the consequences 

 of the hypothesis of the origin of anorthosite is apparently rather 

 good. The Norwegian and the Russian areas are equally signifi- 

 cant, but no attempt will be made to discuss them in detail. Refer- 

 ence will be made, however, to the schematic presentation of 

 differentiation given by Kolderup, which is based entirely on field 

 evidence, and of which a copy is presented below. Attention 

 is called to the central position of the norite with its anchi- 

 monomineralic basic differentiates and its more complex acid 

 derivatives. These are, it is believed, the accumulations of sorted 

 crystals on the one hand, and the residual liquids on the other. 



1 Ibid., p. 122. 2 Ibid., p. 124. * Ibid., p. 126. 



4 A dike of anorthosite in the Cripple Creek country, to which the writer's atten- 

 tion was called by Professor Graton as an apparent exception, is described as contain- 

 ing biotite and quartz. It evidently varies toward granite, and its occurrence as a 

 dike might reasonably be expected. 



