'94 NEW YORK STATE MUSEUM 



though it is not frequent in the Adirondacks in the writer's ex- 

 perience. Certain very garnetiferous, hornblende gneisses are 

 very frequently associated with the limestone, though there are 

 many other rocks rich in garnets, notably many gabbroic gneisses, 

 so that such beds are by no means diagnostic of the series in the 

 absence of others. In fact it is quite probable that much, if not 

 all of such gneiss, when in association with the limestones, has 

 been formed from gabbro intrusions. 



These characteristic Grenville rocks are interbanded and inter- 

 folded with other gneisses which are precisely like rocks that 

 occur over wide areas in which all trace of the Grenville rocks is 

 absent. These are various, mostly fine grained, granitic, syenitic 

 and gabbroic gneisses, so called since they have the mineralogy 

 and composition of these igneous rocks, and have already been 

 referred to as gneisses of doubtful age and origin. Some of these 

 rocks associated with the limestones are so finely granular that 

 they strongly resemble sandstones of various colors in the field, 

 l)\it the microscope always dispels this impression, absolutely no 

 indication of clastic structure being visible, while the mineralogy 

 is that of the igneous rocks above noted. In the writer's ex- 

 perience an abundance of these fine grained rocks is fairly indic- 

 ative of the presence of the limestones near at hand, at least 

 they have not been met with abundantly away from them. But, 

 if this fineness of grain be a constant difference,, it is the only one, 

 and such rocks are provisionally regarded as the equivalents of 

 the other gneisses, representing dikes and sheets injected into 

 the Grenville rocks which have been given a parallel foliation and 

 .an interbedded appearance by subsequent metamorphism. Their 

 more finely granular structure may perhaps be accounted for as 

 ;a result of their proximity to the limestones, and to the yielding 

 character of the latter under the severe pressures producing the 

 metamorphism. 



It is not to be supposed that anything like all the patches of 

 the Grenville series are represented on the map. 1 The country 



lThe Malone and Waverly patches undoubtedly are of much greater 

 extent than is indicated on the map. To the southwest of each is a wide 

 -•area of fiat country with no outcrops, indicating Grenville rocks below. 



