130 MEW YORK STATE MUSEUM 



per cent; weight for each cubic foot 163.3 an< ^ x 6i pounds; per- 

 centage of water absorbed .30. The ratio of absorption and poros- 

 ity are considerably higher than the figures obtained on the Yonkers 

 gneiss of the Hackett quarries, which are given elsewhere. 



THE HARRISON DIORITE 



The Harrison diorite covers an area of several square miles 

 within the towns of Mamaroneck, Rye and Harrison, Westchester 

 county. It forms two nearly parallel belts striking northeast and 

 southwest, of which the easterly one extends along the sound from 

 Port Chester to Milton Point and the westerly one, 2 or 3 miles 

 inland, from the Connecticut line to near Larchmont station. The 

 belts are only about a mile wide at most and show intrusive con- 

 tacts with the Manhattan schist. Across the Connecticut border, 

 they unite with a large area of the same rock that is known there 

 as the Danbury granodiorite. 



The rock has a well-marked gneissoid texture, which indicates 

 that it was intruded before the igneous and sedimentary formations 

 of this section were metamorphosed. The date of the intrusion, 

 therefore, is earlier than the period of folding that came at the 

 close of the Paleozoic and later than the Manhattan schist. The 

 diorite resembles in composition the more acid members of the 

 Cortlandt series, but its foliation indicates a separate and prior 

 period of formation, for the Cortlandt rocks are practically un- 

 changed. 



Strictly speaking, the rack is a granodiorite, as in its general 

 development, it shows affinity with the granites through the presence 

 of quartz, and considerable alkali-feldspar. The quartz is in fine 

 grains and has a smoky color. The feldspar includes a white 

 plagioclase of andesine to labradorite composition and a nearly 

 colorless microcline. Besides the fine granular feldspar of the 

 groundmass, there are quite frequently porphyritic individuals which 

 have been compressed into lenses or augen. These are made up of 

 twin crystals. They measure up to an inch or so long and half that 

 in width, but are more commonly of smaller dimensions. The 

 longer axis and the twinning planes are parallel to the rock foliation. 

 Biotite is the chief ferro-magnesian constituent, but is supplemented 

 by a little hornblende. The biotite is plentiful, in scaly aggregates 

 that interleave the quartz and feldspar. Parallel to the foliation 

 thus produced, the rock breaks more or less readily and the result- 

 ing surface is always much darker than the fractures across the 

 foliation. Of smaller importance is garnet which appears in 



