RlES — OxEO'LOGY OF OrATTGE COUNTY. 



Dike Rocks. The only detailed account hitherto published of dikes 

 from the Highland area in Orange county, are two papers by Professor 

 Kemp which appeared in 1888"", both treating of camptonite dikes, which 

 at that time were thought to be comparatively rare rocks. The tirst of these 

 is a camptonite from the north end of the first railroad cut above Fort Mont- 

 gomery. It is a dense black rock, and the dike extends twenty feet or more 

 vertically, running diagonally across the lamination of the gneiss. Under 

 the microscope the rock consists of small, well developed hornblende crystals 

 with sharp faces. There is some plagioclase and a few grains of magnetite, 

 and the dike has a, more or less porphyritic structure. Duplicate analyses 

 of this rock, which are of value, were made by Professor Dennis*, of Cornell 

 University. They show: 



a. b. 



Si a 44.85 44.87 



Al 2 3 ' 17.20 17.281 



Fe 2 3 11.20 11.04 



FeO . 



Mn O . trace trace 



Ti 2 . . . . . . 6.578 6.738 



Ca O 7.52 7.54 



Mg O 5.02 4.946 



K 2 O . 2.992 2.621 



Naj O . ; ... . 1.390 1.611 



P 8 5 383 .447 



C 2 



Loss on ignition 2.387 2.491 



99.52 99.585 



There are no dikes between Fort Montgomery and Crow 's Nest mountain, 

 but extending up the face of the latter are six dikes which have been noted 

 by Professor Kemp. They are holocrystalline aggregates of hornblende, 

 augite and plagioclase with subordinate magnetite, apatite and biotite, and 

 sometimes orthoclase and quartz.f 



The hornblende and augite are generally associated, but may occur singly: 

 the augite especially in spots where the dynamic action has been greatest. 



* A Diorile Dike at the Forest-of-Dean mine, Orange county, N. Y.—A. J. S. (3), xxxv p. 331 and 



The Dikes of the Hudson River Highlands— Amer. Nat. xxii p 691. 

 t For a review of other occurrences of camptonitic rocks and their classification, see Hull. 107. V. S. (}. tf.-The Dikes of Lake 

 Champlain by. J. F. Kemp and V. F. Marsters. 



