r36 NEW YORK STATE MUSEUM 



regarded as of Grenville age on account of the association of a 

 similar gneiss with the undoubted Grenville rocks east of Colby 

 pond. The syenite is in thin sheets intruded along the joints. 

 It contains only sporadic quartz and is a rare rock in the 

 Adirondacks. 



Only a few yards south of the reservoir the contact between 

 the anorthosite and what is taken to be the same gneiss is 

 exposed (973). The contact is sharp, and the anorthosite includes 

 great lumps of the gneiss and is unquestionably the younger rock. 

 The gneiss differs from that at the reservoir in containing hypers- 

 thene in addition to augite and hornblende so that it is a gabbro 

 norite rather than a gabbro diorite, but all such gneisses in the 

 Adirondacks show rapid mineralogic change within short dis- 

 tance. The intruded syenite is also lacking here, but the distance 

 from the reservoir is so slight that it is thought to be certainly 

 the same gneiss. The anorthosite at the contact has still 90fo 

 of feldspar, but in addition to the labradorite, which gives 

 slightly lower extinctions than usual, barely reaching 20^, there 

 is considerable antwinned feldspar which consists of a blotchy 

 intergrowth of orthoclase and albite, the intergrowths having 

 none of the regular appearance of the usual microperthite. This 

 rock represents, then, an intermediate stage between the usual 

 granular anorthosite norite of the boundary and the dike rock 

 from the second cut of the previous section, in which the feldspar 

 is wholly of this type. This same rock is also met elsewhere in 

 the district. 



Summary of section. This section is at right angles to, instead 

 of parallel to, the boundary, and shows a passage from normal 

 anorthosite to granular anorthosite gabbro as the boundary is 

 approached. It also exhibits a contact against the adjacent 

 gneisses, which shows the anorthosite to be the younger. It 

 farther shows rocks intermediate between anorthosite and such 

 microperthite rocks as were described in dike form from the pre- 

 ceding section, indicating their close relationship to the anor- 

 thosite in origin and in time. 



