80 NEW YORK STATE MUSEUM 



order to establish reasonably sharp conceptions. In typical ex- 

 posures all these are exhibited with all desirable clearness. Thus 

 there is never any doubt about the crystalline limestones of the 

 Grenville; and the associated schistose rocks, quartzites, and thinly 

 foliated, rusty gneisses are almost equally easy to identify. Re- 

 peatedly some little exposure of the last named has attracted at- 

 tention and has led to the later discovery of anticipated limestones. 

 But as more massive gneisses are met the difficulties of drawing 

 the lines of demarcation increase. No geologist can escape great 

 uncertainty of mind in these regards. The writer has endeavored 

 to set some of these fully and frankly forth in the subsequent dis- 

 cussion of the stratigraphic relations of the iron ores, and in the 

 details of the Port Henry area of the Grenville. From long resi- 

 dence upon the metamorphosed sediments represented by the mica 

 schists of Manhattan Island and familiarity thus acquired with 

 undoubted rocks of this t3^pe on the one hand, as well as study 

 of the Adirondacks and Highlands, as representatives of intrusive 

 types, on the other, a disposition has been acquired to look for 

 decided and fairly regular schistosity as an indication of sedi- 

 mentary origin and in questionable cases, seeing that we are dealing 

 with an undoubted plutonic district, the irresistible tendency is to 

 place the massive gneisses of composition corresponding with in- 

 trusives, in the category of the eruptive rocks. Upon the map 

 therefore in not a few cases the heavy massive gneisses have been 

 put with the syenite series, with which also their mineralogy allies 

 them, and with whose typical representations they are connected 

 by imperceptible gradations. Yet it is quite conceivable that another 

 observer might reach a quite different conclusion. 



Another very troublesome difficulty arises in this further par- 

 ticular. From the typical representatives of the anorthosite, 

 syenite and gabbro series, there are variations. Anorthosites in 

 characteristic exposure contain little else than labradorite or some 

 related plagioclase, and of this type there is no lack. But dark sili- 

 cates appear in greater and greater amount; orthoclase does not 

 entirely fail ; and gneissoid structures are produced by the omni- 

 present crushing and shearing. In the end gneisses result, largely 

 dark silicates, yet with augen or knots of blue labradorite still dis- 

 tinctly visible. Thus the Split Rock and Woolen Mill types re- 

 sult, both of which are intrusive in anorthosite. Yet the inter- 

 mediate stages are well developed, especially along the outer border 

 of the anorthosite mass and after much uncertainty as to the best 

 course it was decided to color the basic anorthositic border as has 



