22 NEW YORK STATE MUSEUM 



which is richer in biotite, hornblende and pyroxene than the* 

 usual syenite. Metamorphism has concentrated these minerals- 

 along certain planes, producing a marked gneissoid structure, and 

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

 enriched by these minerals the rock approaches more nearly to a 

 gabbro in composition, otherwise 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 syenite. Bands of very basic feldspar, hornblende, 

 biotite gneisses occur frequently with the others, and the whole 

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

 Rocks 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 garnets 

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

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

 diate 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 with these at times also. The 

 whole series is perplexing and uncertain. The rocks differ 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 which 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 metamorphosed. 



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 anj* definite answer. If they are equivalent, it 

 is strange that the pronounced porphyritic character of the rock 

 of the two outliers should have so utterly disappeared in the main 

 mass. Yet it may be legitimately argued that the porphyritic 



