l6 NEW YORK STATE MUSEUM 



GENERAL GEOLOGY 

 Introductory and Historical 



The variety of rock types in the Highlands has been recognized 

 since the beginning of the study of geology in America. Amos 

 Eaton in his Index to the Geology of the Northern States says in 

 speaking of this district, " I believe every known variety of granite 

 is found here." The writers are convinced that this was a fair 

 approximation to the actual facts. 



The first careful study of the region was made by W. W. Mather 

 in 1843.- In spite of the short time allowed for the completion of 

 the work, necessitating the covering on an average of 30 square 

 miles a day, not oaily were the broad general features recognized 

 and described but a remarkable amount of local detail was collected. 

 When the different use of technical terms is allowed for, his state- 

 ment of the causes underlying the complexity of the country is seen 

 to indicate a remarkable insight into the more difficult and obscure 

 phases of Precambrian metamorphic and igneous geology, and is on 

 the whole strikingly similar to the most recent conceptions. He 

 seems to have regarded the region as made up of sedimentary strata 

 metamorphosed by intrusives of great penetrating power. He con- 

 tinually mentions " the granite laminated among the limestone 

 strata" (p. 484). "the granite interstratified with the gneiss" 

 (P- 525), "the greenstone which is intruded in sheets and irregu- 

 lar masses among the gneiss and other rocks in the same way as 

 granite and syenite" (p. 532). 



He noted the association of the hornblende gneisses with the 

 magnetite veins, " the hornblendic rocks are constantly associated 

 with the beds of magnetic oxide of iron which are so numerous in 

 the Highlands " (p. 534), and understood the true igneous character 

 of the veins, " They form masses in gneiss and hornblendic gneiss 

 rocks, which by casual examination would be called beds, but after 

 careful examination of the facts I think they may be called veins. 

 . . . They lie parallel to the layers of rock, but by close examina- 

 tion it is found in many instances after continuing this parallelism 

 for a certain distance, the ore crosses a stratum of rock, and then 

 resumes its parallelism, then crosses obliquely another and so on. 

 ... In other places where a great bed of ore occurs at some 

 depth, only a few small stripes of ore penetrate through the super- 

 incumbent mass to the surface, as if the rocks had been cracked 



^ Mather, W. W. Geology of New York. Part I, comprising the geology 

 of the First Geological District, 1843. 



