Williams — Gabbros an d Diorites of the Cortlandt Series. 443 



All the other constituents of the coarse hornblende-mica- 

 diorites of Cruger's Point are identical with those of the typi- 

 cal and more abundant mica-diorite. They may therefore 

 best be described in connection with this rock (class V), of 

 which indeed the type now under consideration is only a par- 

 ticular facies. 



A very fine-grained variety of the hornblende niica-diorite is 

 quite common as a dike rock on both Montrose and Stony 

 Points. In many respects this presents a resemblance to Rosen- 

 busch's group of dioritic lamprophyres or kersantites, and yet 

 its extreme freshness and freedom from calcite, the frequently 

 granular form of the feldspar, and the association in nearly 

 equal proportions of biotite and green hornblende while augite 

 is wholly wanting, separate this rock from any of the many 

 varieties of kersantite described in Rosenbusch's recent work. 

 The fine-grained, dark-gray dikes of this rock may be most 

 advantageously seen in the cuttings on the West Shore Railroad 

 through and near Stony Point. Here they intersect the much 

 contorted schists, the peridotite and the mica-diorite and afford 

 the evidence upon which Professor Dana admitted the truly 

 eruptive nature of at least the more basic members of the 

 " Cortlandt Series." * These dikes are, however, shown by 

 the microscope to belong to the more acid rather than to the 

 more basic of the massive rocks. 



Class V. Mica-Diorite. 



This rock is more uniform in its character than any other of 

 the important members of the Cortlandt Series. It is in all 

 cases essentially a rather coarse-grained aggregate of plagioclase 

 and biotite, with accessory epidote, apatite and magnetite ; often 

 a little orthoclase and quartz ; and sometimes garnet. The 

 latter mineral is an endo-metamorphic product, and is to be 

 found only near the contact with the schists. 



The mica-diorite occurs only in the south-western part of the 

 Cortlandt area ; on the east side of the Hudson River west of 

 Cruger's station, and on the west side at Stony Point (see map 

 in my paper on the Cortlandt Peridotite, this Journal, Jan., 

 1886, p. 29). 



No such pure type of mica-dioritef has ever, to my knowl- 

 edge, been described from any locality. 



* This Journal, III, xxviii, p. 384, Nov., 1884. 



f Professor J. D. Dana called this rock soda -granite, (this Journal, III, xx, p. 

 198), and later hemi-dioryte, (lb., xxv, p. 478). The first name was proposed by 

 Haughton in 1856 to designate, as it still does, a true granite in which the soda 

 is in excess of the potash, (cf. A. Gerhard, Neues Jahrb. fur Min., etc., 1887, II, 



Am. Jour. Sct. — Third Series, Vol. XXXV, No. 210.— June, 1888. 

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