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THE 



AMERICAN JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



Art. XXX. — The columnar structure in the igneous rock on 

 Orange Mountain, New Jersey ;* by JOSEPH P. IDDINGS, of 

 the U. S. Geological Survey. (With Plate IX.) 



The long ridge lying west of the City of Orange, 1ST. J., and 

 locally known as the Orange Mountain, is formed of beds of 

 older Mesozoic sandstone, dipping gently to the northwest. It 

 is capped by a layer of igneous rock variously designated as 

 "trap," diabase or dolerite. The same rock forms the Palisades 

 on the Hudson and occurs over large areas in Connecticut and 

 other Eastern States. The thickness of this particular layer 

 varies considerably, but in the vicinity of Orange it averages 

 about 100 feet. Along the east face of the ridge it is exposed 

 as a cliff whose lower portion is concealed under the angular 

 fragments into which the rock readily breaks ; but numerous 

 quarries have been worked in it to obtain the material for 

 building the excellent roads which traverse the neighboring 

 country. These quarries, many of them of long standing, give 

 us in section the structure of this lava sheet. In nearly all of 

 them a more or less distinctly columnar structure is noticeable 

 in some part of the exposure, usually the upper portion. Two 

 localities are of special interest, because of the size and perfec- 

 tion of the columns in one case, and of their arrangement in 

 converging groups in both places. 



The first is John O'Rourke's quarry on Mt. Pleasant avenue, 

 just south of Llewellyn Park. This has lately become quite 



*Read before the Philosophical Society of "Washington, June 6th, 1885. 



Am. Jour. Sex— Third Series, Yol. XXXI, No. 185.— Mat, 1886. 

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