THE ANORTHOSITE BODY IN THE ADI RON BACKS 513 



irregular, as Bowen suggests, so that the syenite differentiate was 

 in separate masses instead of in a continuous sheet, there should 

 still remain considerable masses of syenite within the chilled border 

 and underlying it, if this border was an upper instead of a lateral 

 one. We should find gabbro passing into syenite and this into 

 anorthosite. I know of no such syenite masses within the gabbro 

 border anywhere in the region; and to explain their absence we 

 should be, it seems to me, forced to the conclusion that, owing to 

 disturbance during a late stage of consolidation, every particle 

 of this syenite differentiate in the upper part of the body was forced 

 out through the roof to higher levels, letting the chilled gabbro 

 down upon the anorthosite. This is perhaps possible, but certainly 

 very unlikely, and, moreover, it would leave unexplained the usual 

 slow and even gradation from border gabbro into anorthosite 

 gabbro and of this into anorthosite as we recede from the border — 

 a feature which seems to me convincingly to suggest a lateral border 

 rather than an upper one. 



In the Palisade sheet, utilized as an illustration by Bowen, the 

 acidic types lie directly underneath the upper chilled border. In 

 the Adirondacks they lie without and above rather than within 

 and below this border, as they should do on the sheet conception. 

 None of the outlying masses of syenite known to me show any 

 sign of a chilled border of gabbro as if they were upper parts of a 

 single large body. They all seem rather of the type of injections 

 upward from some large mass of magma below. It is to be under- 

 stood that I am not objecting to the laccolithic conception, but 

 to the conception of a single laccolith occupying the entire region. 

 If we regard the anorthosite mass as a single mass, its margins 

 shown by the chilled border, attacked shortly after its formation 

 by masses of syenite magma, which arose from one or more separate 

 and deeper bodies to the west and which came up along the margin 

 of the anorthosite, not through it, we obtain an explanation of the 

 abrupt transition from anorthosite to syenite territory which ob- 

 tains in the region and we are free from the difficulties which have 

 just been discussed. 



Dr. Bowen's contention that the present-day ideas in regard 

 to the time relationship between syenite and anorthosite are chiefly 



