512 H. P. CUSHING 



of opinion, it still seems to me to be advisable to keep an open 

 mind on the possibility that the syenite and anorthosite occur 

 "substantially as layers with the syenite above." It is especially 

 desirable in view of the fact, recognized by Professor Gushing, that 

 the anorthosite occurs in the more deeply eroded portions and the 

 syenite principally at higher horizons, an arrangement not easily 

 reconciled with the opinion that the syenite pushes up into the 

 anorthosite from below. 



ADIRONDACK INTRUSIVES 



H. P. CUSHING 



I am greatly indebted to Dr. Bowen for his additional contribu- 

 tion to this discussion. I take the liberty of considering briefly 

 the points he brings out. 



He suggests that the chilled gabbro border of the anorthosite 

 is not a lateral border but a remnant of an upper one. It is very 

 difficult for me clearly to visualize the structure of the region on this 

 view. It is, roughly, about ioo miles across the mid- Adirondack 

 region from east to west, and, again roughly, the easterly half of 

 this distance is occupied by pretty clean anorthosite, and the 

 westerly half contains a great number of syenite bodies and no 

 anorthosite at all. The chilled gabbro border is about midway 

 of the region. If it is a chilled upper portion of a laccolith, consist- 

 ing of pyroxenite and gabbro below, then anorthosite, then syenite, 

 and, finally, the chilled gabbro roof, since tilted so that the present 

 erosion surface cuts it at a considerable angle, it is necessary to 

 conceive that this chilled upper surface passes below ground in 

 the westerly direction and into the air to the east. Under such a 

 view the present-day syenite masses of the west must have broken 

 through this cover to reach their present position, and there is no 

 particular difficulty in imagining that they did so. But under 

 this view it seems to me necessary that we should also find syenite 

 to the east of the chilled border and close to it — that syenite which 

 formed as a differentiate in the upper part of the chamber, under- 

 neath the chilled upper surface. Even if the upper part was very 



