OF PRE-CAMBRIAN ROCKS 197 
in anorthosite, of somewhat gabbroic nature for the most part. Pan- 
ther Mountain, at the south end of Upper Saranac Lake, which was 
climbed by the party because of the good view of the general topog- 
raphy of the region obtainable from its summit, furnished a good 
exhibition of the general character of the anorthosite. Syenite dikes 
of various sizes persist in the anorthosite with diminishing frequency 
for a distance of three or four miles from the edge of the mass. 
The next day boats were taken at Axton for a trip down the Ra- 
quette to Tupper Lake, with a side excursion into Follensby Pond. 
The anorthosite was found grading from the normal rock at Axton 
into a basic gabbro near Follensby Pond, adjoined by the syenite 
in somewhat basic phase, illustrating the general tendency of the 
anorthosite bathylith to become more basic at its borders, and the 
similar tendency of the syenite to become more basic in the vicinity 
of the anorthosite. At the upper end of Follensby Pond Grenville 
rocks were seen, the outcrops being at the northern extremity of a 
long belt of these rocks extending unbroken for several miles. Lime- 
stone, both pure and impure, rusty graphitic gneiss, and a little 
quartzite were seen. ‘These and other rocks of the belt are precisely 
like those seen the preceding week and those seen later in Canada. 
Down the river from Follensby the rocks seen were anorthosite of 
the gabbro type followed by syenite. 
The rocks of the central area of the Adirondack region consist 
mainly of bathyliths and smaller masses of anorthosite, syenite, 
granite, and gabbro, with only occasional patches of the sediments, 
and the region seen in the three days was a good sample of the general 
area. The asymmetric differentiation of the syenite into a basic 
type in the vicinity of the anorthosite, which it cuts, and into an acid 
type in the contrary direction where it adjoins and cuts granitic 
gneisses, as well as the local differentiation of the acid syenite into 
the basic type around an inclusion of anorthosite, seemed to point 
to the digestion of material from the adjacent rocks as a cause of 
the differentiation. 
Monday, July 16, was mainly taken up in journeying from Tupper 
Lake to Theresa in Jefferson County, to see there the rocks as they 
appear in the northwestern region. The evening of July 16 and the 
whole of the next day were spent in this examination. A thick 
