26 NEW YORK STATE MUSEUM 



syenite or granite and Grenville gneisses in about equal amounts. 

 There are few suggestions of assimilation, the igneous and sedimen- 

 tary rocks generally retaining their characteristic features. 

 The other mixed gneiss areas require no special mention. 



THE GABBRO AND ITS DERIVATIVES 



Because of the large number of exposures, mode of occurrence, 

 excellent outcrops, frequent contacts against the country rocks, 

 and the remarkable variations in composition and appearance the 

 gabbros are of unusual interest and will be described in considerable 

 detail. 



Mode of occurrence 



The gabbro and its derivatives nearly always occur in the form 

 of small stocks or bosses rather than as true dikes, their length 

 ranging from 30 to 40 feet to about a mile, and with widths up to 

 three-fourths of a mile. Sixty-one separate gabbro bodies were 

 found and are shown on the geologic map. In spite of the detailed 

 field work a few others have probably escaped the writer's notice. 



The ground plan, as represented on the geologic map, is almost 

 invariably elliptical, though sometimes approaching the circular. 

 When the contact with the country rock is carefully traced out it 

 is commonly found to be sharp and shows smooth or flowing out- 

 lines against the country rock. In only two or three cases do the 

 gabbro masses approach the true dikelike form, and in each of these 

 cases fine-grained tongues were found to extend into the surround- 

 ing rock. One stock, one and one-third miles southeast of Chester- 

 town, shows several such tongues, one of them (i to 6 inches wide) 

 clearly cutting the granite porphyry for 30 feet. Other and smaller 

 gabbro dikes at the summit of Hackensack mountain, and i mile 

 south of South Horicon show a number of such fine-grained tongues. 



At one place in the dike or boss which crosses the road 1^4 niiles 

 a little west of south of South Horicon, fairly coarse-grained gabbro 

 is in sharp contact (for 6 or 8 feet) with fine-grained gabbro, the 

 latter becoming coarser grained again away from the contact. This 

 suggests a second intrusion of the gabbro after the first but after 

 the first had cooled. 



It is a striking fact that in spite of many excellent contacts which 

 were observed, such dikelike tongues are so rare. As Harker says ^ : 

 "Although most of the bodies of granite and other plutonic rocks 



^ Natural History of Igneous Rocks, 1909, p 86. 



