22 NEW YORK STATE MUSEUM 



which is richer in biotite, h'ornblende and pj^roxene than the- 

 (isnal syenite. Metamorphism has concentrated these minerals^ 

 along certain planes, pr'oducing a marked gneissoid structure, and 

 a rock varying from green to black in general color. Where thu» 

 enriched by these minerals the rock approaches more nearly to a 

 gabbro in composition, other^vise it is a syenite, though often very 

 quartzose. With the increase in the quartz percentage the color 

 often changes with facility from green to red and back again^ 

 just such a color change as is often seen in the great Adirondack 

 masses of S3'^enite. Bands of very basic feldspar, hornblende, 

 biotite gneisses occur frequently w'ith the others, and the whole 

 series is cut by a multitude of small veins of quartz and pegmatite. 

 E'ocks like these appear in a multitude of exposures. All have the 

 mineralogy of igneous rocks and are believed to be such. 



Along with these a rock repeatedly occurs which resembles some- 

 what the green and black gneiss of the above, but contains garnet* 

 numerously also. It has the mineralogy of a rather basic igneous- 

 rtock, neither a gabbro nor a syenite however, but of a rock interme- 

 dircte between them and known as monzonite. It passes on the one 

 hand into the syenite gneisses and on the "other into the darker 

 colored gneisses, often with graphite, of the Grenville series, the 

 lighter colored gneisses appearing v/ith these at times also. The 

 whole series is perplexing and uncertain. The rocks ditfer con- 

 siderably from the red, black and green gneisses previously de- 

 scribed and regarded as igneous rocks of Grenville age. A possi- 

 ble, and perhaps the simplest explanation of the whole is that it 

 is a sort of border belt between the syenite and the Grenville rocks,, 

 in Avhich these last were all cut up by the syenite intrusion and 

 in which they now occur as patches. Many contact rocks were 

 formed and the whole subsequently was severely metamor]ihosed. 



There still remains the question of the relationship of this 

 syenite to that at Little Falls and Middleville, and to it the writer 

 is unable to give any definite answer. If they are equivalent, it 

 is strange that the pronounced porphyritic character of the rock 

 of the two outliers should have s^o utterly disappeared in the main 

 mass. Yet it may be legitimately argued that the porphyritic 



