THE PROBLEM OF THE ANORTHOSITES 227 



anorthosite. It is rather a balance between density, rate of cooling, 

 and viscosity such that the necessary amount of sorting of crystals 

 occurs. 



The writer must confess an inability to state precisely the reason 

 why the species presented in an igneous sequence at one locality may 

 be different from those at another. It is nevertheless believed that 

 it is unnecessary that the original magmas need have been different, 

 or even that the manner of differentiation need have varied. The 

 results seem to be possible if there was a variation in the extent to 

 which separation of crystals from liquid and also sorting of indi- 

 vidual minerals were able to take place. Variations in these factors 

 depend on physical conditions which have, however, their chemical 

 consequences, for the removal or non-removal of a crystal has each 

 a perfectly definite effect on the future course of the liquid. 



In one sequence, which is well shown in the pre-Cambrian of 

 Ontario and in certain British intrusives, there is practically only 

 gabbro and granite with little that could be described as inter- 

 mediate. Apparently this is especially likely to be true of masses 

 of moderate size. In somewhat larger masses ultrabasic rock may 

 make its appearance as one of the members with occasionally some 

 anorthosite. Usually for the formation of anorthosite a very large 

 mass is necessary, and possibly also a deep-seated mass. On the 

 other hand, for the formation of those sequences that emphasize 

 intermediate types such as diorite, quartz diorite, and granodiorite 

 the indications are that very large masses are necessary, but that 

 they should probably occur at moderate depths. There is nothing 

 here in the way of hard and fast rules, but there do seem to be 

 fairly definite tendencies. All of these are reasonably to be con- 

 sidered the result of differences of the physical conditions under 

 which cooling took place. 



THE MORIN ANORTHOSITE 



General features. — The Morin anorthosite area of Canada is in 

 many respects very like the Adirondack area. It lies near the edge 

 of the great pre-Cambrian shield where it is overlapped by Paleo- 

 zoic rocks. It covers a territory of about 1,000 square miles which, 

 while not as mountainous as the Adirondacks, is nevertheless quite 



