ROGERS, GEOLOGY OF THE CORTLANDT SERIES 85 



The technology of corundum is discussed by J. H. Pratt. 83 From the 

 composition of the Cortlandt emery as described above, it is evident that 

 it is low grade ore. The presence of spinel, sillimanite, garnet, feldspar, 

 quartz, etc., greatly lowers its cutting efficiency and materially affects its 

 toughness. At the same time, the presence of spinel, which is 8 in hard- 

 ness, would not interfere with the manufacture of a vitrified emery 

 wheel; and for many purposes where extreme hardness is not required, 

 the spinel emery furnishes a convenient transition between garnet and 

 true emery. The unsuccessful attempt to smelt the more ferriferous 

 varieties for iron has been noted above. Even earlier than this, it had 

 been suggested 84 that the ore might be used as a refractory lining for the 

 puddling furnace, open hearth and Bessemer converter, as well as an 

 aluminous flux in the blast furnace in admixture with a siliceous stock, 

 but apparently these sugestions have not proved practicable. 



Summary 



The Cortlandt Series is an igneous complex situated just southeast of 

 Peekskill, N. Y., and is about 28 square miles in extent. Its correlation 

 cannot be definitely settled, although it is probably late Paleozoic. The 

 rocks have been described by J. D. Dana and G. H. Williams, the former 

 attacking them from the standpoint of genesis, and the latter treating 

 them rather as an aggregation of petrographic types. In the present 

 paper, they are treated more from a geological standpoint, the differen- 

 tiations made in the various broad types being directly proportionate to 

 their areal importance. Eeference to the appended map will impart the 

 distribution and extent of the varieties determined, and on page 58 is 

 given a diagram which indicates the most salient relations between the 

 several types. The various differentiations of the norite magma are 

 most centrally located; they are flanked on both sides by pyroxenites 

 which extend to the west, and an unknown distance under the river, pos- 

 sibly as far as Stony Point, where they again outcrop. Between the 

 norites and the western area of the pyroxenites lies a diorite area. The 

 granites are more isolated, though unquestionably part of the series. 

 The most basic members at least grade into one another in many cases, 

 while at times, sharp contacts have been found. The analyses of the 

 more important types indicate an unmistakable serial relationship, 

 although complicated by the entrance of the pyroxenites. It is probable 

 that the latter were intruded first, followed closely by the norites, so that 



» Bull. 269, U. S. G. S., p. 159. 1906. 



84 J. P. Kimball, Amer. Chemist, IV, 321. 1874. This paper contains four analyses of 

 the emery. 



