48 NEW YORK STATE MUSEUM 



be referable to the basic syenites and not necessarily to the gabbros, 

 as formerly believed. Field associations and preponderance of 

 orthoclase as seen under the microscope must hereafter decide. 



On weathering, the syenitic rocks are particularly prone to de- 

 velop a rusty exterior although just why the contained iron is in 

 a condition so sensitive to alteration, is not apparent. When one 

 seeks for a fresh hand specimen from fallen blocks, it is often 

 necessary to pound off several inches or the better part of a foot 

 before the fresh green rock appears at the core. Where polished 

 Q& smooth and hard by glaciation, the rock may also develop a 

 very white coat or skin which, however, is easily chipped off so 

 as to expose the fresh green beneath. 



The relative proportions of the several minerals vary widely over 

 extended areas. Toward the acidic extreme, quartz may become 

 increasingly abundant, fully enough to carry the rocks over into 

 the granites. Such varieties appear on Barton hill near Mineville. 

 and in association with the ores of the great mines. Yet the 

 microperthitic character of the feldspar is pronounced and the same 

 augite and hornblende are in evidence which we find in the typical 

 specimens, so that one can not well avoid the conclusion that there 

 is a fundamental relationship. 



Very instructive evidence has been afforded by the numerous 

 diamond drill cores which have been obtained in the explorations 

 for magnetite at Mineville. The writer has examined most care- 

 fully thousands of feet of these, and finds on the whole the average 

 syenite most frequently present, but with no evidence of being a 

 separate intrusive mass. The most acidic variety will quite sharply 

 replace it; and in the same way a very basic variety may come in 

 and constitute the section for 50 or 100 feet or more. Yet while 

 the transition is sharp there is no evidence of separate intrusive 

 masses nor is one justified in inferring more than a differentiation 

 of an eruptive mass into layers or portions of contrasted com- 

 position. As will be later shown in speaking specially of the ore,-, 

 the great body of magnetite lies immediately beneath the most 

 acidic phase. The ore contains appreciable amounts of the emerald- 

 green pyroxene and is simply an extremely basic concentration of 

 one of the normal minerals, the magnetite, accompanied by two 

 others, the pyroxene and apatite. Beneath the ore, usually if not 

 always, is found a basic phase of the syenite. All these relationships 

 will be more fully discussed from the standpoint of the ore on 

 subsequent pages, the object being at the moment to emphasize the 

 variability of the rock mass. 



