26 ' NEW YORK STATE MUSEUM 



Almost every variety of igneous or metamorphic rock that could 

 be obtained by the differentiation of granite magmas and their 

 attack on country rock is to be found here. And there have been 

 several magmatic invasions, each making its own particular addition 

 to the existing confusion. The more ancient gneisses, schists and 

 limestones, already complex and obscure, are made even more so by 

 contact influence and by magmatic and mineralizer impregnation of 

 most elaborate character. This latter has served to mix, in hope- 

 less confusion, constituents of original sedimentary with con- 

 stituents of later igneous origin into a rock that is neither the one 

 nor the other, but which can be understood as essentially an impreg- 

 nation gneiss. There are also the banded types which are, in part 

 at least, lit- par-lit injection of simple igneous matters along the 

 weaknesses of the invaded rocks, all of which had been previously 

 metamorphosed and may already have been impregnated. Suc- 

 cessive attacks of this sort are not rare and a particular outcrop may 

 contain representatives of practically all the igneous members of the 

 Precambrian series. 



Under these circumstances, quartzites may not be distinguishable 

 from gneisses nor determinable as to igneous or sedimentary origin ; 

 and schists may be made from eitlier recrystallized sediments or 

 sheared igneous rocks. Limestones may be so changed by subtrac- 

 tions, additions and reorganizations of their constituent minerals as 

 to be scarcely distinguishable as limestones at all. 



It would be a mistake to assume that all the formations are so 

 confused or that they all show such indeterminate penetrations and 

 gradations. It is somewhat less characteristic of the Storm King 

 granite than of the other Precambrian granites, but even this is. in 

 some places, quite confused. The Grenville also, especially those 

 beds in which limestones are prominent, seems to be readily distin- 

 guished. But one needs little experience to become convinced that 

 even this simplicity is more apparent than real, for it is frequently 

 impossible lo or 15 feet away to determine whether one is dealing 

 with an old sediment or an igneous rock. As far as appearances and 

 structure are concerned, it might be either. 



The chief types of rocks. It thus happens that one may list 

 an exceedingly large number of rock species from these members of 

 the Highlands series — enough to stock a museum of acid igneous 

 rocks of intrusive and plutonic types, and the Avhole gamut of 

 metamorphic rocks. The most abundant types, however, are the 

 following : 



