182 [Assembly- 



clue to the conditions prevailing at the time of deposition, and to their 

 probable origin. The importance of studying these crystalline rocks 

 from the chemical standpoint rather than from that of mineralogical 

 composition is worthy of note, since the first points back to the 

 original sediment; the latter is the result of subsequent conditions 

 prevailing during the time of alteration or metaniorphism. The pres- 

 ence of minerals having definite composition leads to the very com- 

 mon supposition that crystalline rocks are definite compounds also, 

 and in this respect differ from the fragmental rocks. This distinction 

 does not exist, and the former grade into oneanother by imperceptible 

 diflferences; and they are no more definite in chemical composition than 

 the latter class. They are the results from the cooling of liquid masses 

 in the earth crust; or they are the altered products, through the 

 agencies of heat and pressure, of original sediments deposited, as 

 gravel, sand, mud, or as fine precipitates through chemical action.* 



The collections of typical rocks from the Highlands, made in the 

 course of this reconnaissance, are not large enough and do not represent 

 all its out-crops suflficiently to afford tlie proper material for a careful 

 microscopic examination. Another season of field-work is necessary 

 in order to get this material. 



The first geological representation of the Highlands region was 

 made by William Maclure in his geological map of the United States, 

 published in 1817, whereon the rocks of the district were classed as 

 belonging to the Primitive Formation. 



In 1819, S. Akerly described the Highlands as made up of gneiss, 

 granite and micaceous schists and belonging to the Primitive Class.] 



Prof. Amos Eaton, in his " Index to the Geology of the Northern 

 States," published in 1820, refers to the granite in the higher moun- 

 tains of the Highlands flanked by gneisses (pp. 107 and 131-2) ; and 

 in his "Geological Nomenchiture for North America," 1828, he notes 

 the granite at West Point, mica slate at Fort Montgomery, and 

 hornblende rock in Butter Hill as Primitive Mocks. 



Prof. William W. Mather, in his Report on the Geology of the First 

 District, described the rocks of the Higlilands and called them Pri- 

 mary. His report contains a great mass of notes on occurrences and 

 phenomena of out-crops in the district, but without much order or any 

 apparent, broad generalizations. J 



The work of Dr. Percival, State Geologist of Connecticut, lapped 

 over into Westchester, Putnam and Dutchess counties, and he i)laced 

 the rocks on the border in his Western Primary System.^ 



* It is interesting here to refer to a very suggestive and pertinent note by Prof. James Hall 

 in his introduction to Vol. Ill, Part I o{ the Palaontology of aVtw York: " The student 

 from the unaltered rocks has been accustomed to see all the sedimentary strata, presenting 

 the aspect of fine shale or slate, or of sandstone and of strata showing infinite gradations 

 between the slate and sandstone ; intermixtures and interlaminations of the one and the 

 other, and all possible modifications of these two simple materials in the sedimentary de- 

 posits; the admixture of calcareous matter producing calcareous shale and calcareous 

 sandstone, and giving a less or more calcareous character to all the intermediate varieties 

 of these rocks and finally the development of limestone. All these are familiar to him ; 

 and as he approaches the changed forms of these rocks, and sees the beginnings of meta- 

 morphism, and the gradual development of the segregated and crystallized minerals, he 

 still looks upon these rocks in the mass, as strata of shale, sandstone, and the intermediate 

 varieties of rocks made by the mingling of these and the accession of calcareous matter." 



t Oft the Geology of the Iludwn liiver, S. A. Kerly, New York, 1820. 



i Geology of tU First District Nat. Hist, of New York, Wm. W. Mather, 1S42, pp. 516, 

 549. 



§ Report on the Geology of Connecticut, by J. G. Percival, 1842. (See loc. cit.) 



