508 



Report of the 



State 



Geologist. 



3. Interbanded with the other gneisses, dark, basic gneisses occur, ranging 

 from an inch to many yards in thickness, and always parallel to the 

 foliation of the enclosing gneiss. They vary nine}] in grain, but in general 

 are rather coarse, and are distinctly schistose in habit. AVhen studied in 

 thin sections they at once separate themselves into two sharply contrasted 

 varieties, the one having the mineralogy of diorite, the other of gabbro. These 

 gabbroic hands are so precisely like the more basic portions of the gabbro 

 masses shortly to be described, that they should unquestionably be classed 

 with them and will be noted in their proper place. The other, and more 

 common variety is composed of plagioclase and hornblende with smaller, but 

 (jnite constant amounts of orthoclase, biotite and magnetite. Microcline is 

 sometimes present. Quartz may appear in small quantity, and in some 

 instances is found as inclusions in the plagioclase. and there only. Some of 

 these gneisses also carry much green augite along with the hornblende, 

 producing an intermediate rock between the dioritic gneisses and the gabbros. 

 In one specimen there is, instead of augite, a considerable content of an 

 Orthorhombic pyroxene which is regarded as bronzite, the pleoehroism and 

 double refraction being much weaker than in the ordinary hypersthenes of 

 the region. Measured extinction angles indicate an andesine ranging toward 

 labradorite as the common feldspar. 



As has been stated, the mineralogy of the rock is that of a diorite, or, in 

 some cases, of a gabbro-diorite, and its eruptive origin is regarded as highly 

 probable. 



The gneissic series contains the workable deposits of magnetic iron ore. 



Series II The rocks of this series consist of coarsely crystalline lime- 

 stones associated with certain peculiar gneisses and schists quite unlike any 

 members of Series I., and with other gneisses which closely resemble those of 

 that series. But one belt occupied by these rocks exists in Clinton county, so 

 far as known, and in that the exposures are few and meagre so that little 

 knowledge of the group can be obtained here, and the reader is referred to 

 Professor Kemp's descriptions of the numerous better exposures in Essex 

 county.* The Clinton county exposures are in Black Brook township and 

 w ill be described under the geology of that area. 



Series III. This series is constituted of rocks of the gabbro family, 

 which are unquestionably of igneous origin and were intruded into the 

 members of the first two series. The main development of the rocks of this 



* J. F. Kemp. Bulletin (Geological Society of America, Vol. VI., pp. 241-262; and Animal Report New York State Geologist 

 for 1H03, Vol. I , p. 444. 



