514 H.-'P. CUSHING 



due to my observations on the Long Lake quadrangle is correct, 

 but needs some comment in order not to be misleading. The 

 anorthosite district is rugged, wooded, and difficult. Very little 

 of it has been mapped in detail on quadrangle maps. Moreover, 

 the marginal types are weaker and less likely to be well exposed. 

 It is as yet unsafe to argue that the Long Lake phenomena are 

 exceptional because they have not been shown elsewhere. 



I regret that Dr. Bowen did not discuss my suggested explana- 

 tion of the Placid syenite inlier as a remnant of the overlying 

 syenite differentiate of the anorthosite. It is there that he found 

 his rocks transitional between syenite and anorthosite, and it is 

 there that they would be expected, if the inlier is such a remnant. 

 These rocks are not on the border, but are within the mass. If a 

 chilled border is lacking between them and the adjacent anorthosite, 

 then the occurrence would seem most easily explained on this 

 theory. 



In conclusion, then, the relations shown in the field along the 

 western border of the anorthosite seem to me to indicate that the 

 anorthosite is one body and the syenite masses to the west and south 

 belong to one or more separate and slightly later bodies. While 

 I welcome Dr. Bowen's theory of the formation of the anor- 

 thosite and syenite, I can neither agree with, nor see the neces- 

 sity of, his idea that all these intrusives of the region are parts of 

 one single laccolithic body. As I picture it, they must represent 

 separate upwellings from one or more deeper-seated magmas. 

 The western border of the anorthosite marks the line of division 

 of the region into contrasted halves — anorthosite to the east, 

 separate syenite masses to the west; and the particular syenite 

 masses which happened to adjoin the anorthosite differ in no 

 particular from those more remote, except in the one that they 

 have assimilated some anorthosite at the contact of the two rocks. 

 Nothing is to be gained by assuming that the adjoining masses 

 belong with the anorthosite body and by attempting to make a 

 separation between them and those more remote. Such a separa- 

 tion would be purely arbitrary, whereas the other divisional line 

 is sharp and obvious. 



