266 GEORGE C MATSON 



J. F. Kemp, 1 there are four others. Two are just below the old 

 pumping-station, and two are about one-quarter of a mile farther 

 upstream. The two near the pumping-station measure 4 and 9 

 inches respectively; the two above the pumping-station, 3 and 8 

 inches. 



In a quarry at the south end of Hazen Street in the city of Ithaca 

 there are three dikes, ranging from 2 to 6 inches in width. The 

 possibility of these dikes being connected with those near the pump- 

 ing-station in Six-Mile Creek, and also with some of those in Cas- 

 cadilla Creek, should not be overlooked. However, direct proof is 

 wanting, and only the fact that they are all in a line nearly parallel 

 with the direction of the joint planes can be cited as supporting the 

 hypothesis that they are connected. 



The rocks penetrated by the dikes at Ludlowville, Taghanic, and 

 Glenwood are Genesee shales and Portage shales and sandstones. 

 The dikes in the immediate vicinity of Ithaca penetrate rocks belong- 

 ing to the Ithaca Group. 



Megascopic appearance 0} the rocks. — The rock is porphyritic, with 

 a dense black aphanitic groundmass, containing phenocrysts of 

 reddish-brown mica and black fibrous masses having the structure of 

 serpentine. The mica is usually in small glistening scales, with an 

 occasional crystal which has a diameter of one-half inch or more. 

 In some cases crystals of olivine may be seen. With a hand-lens, 

 grains of a black metallic mineral (magnetite) are occasionally found. 

 The phenocrysts of serpentine often cover the surface of the weathered 

 rock as black protuberances, sometimes as much, as one inch in 

 diameter. These resemble pebbles, but are soft, and when broken, 

 show their fibrous structure. 



As the dikes weather they lose their dark color and become a 

 light greenish-gray. On the surface are many mica scales which 

 have faded to a light brown. Further weathering produces a yellow 

 incoherent clay, containing many scales of light brown mica. 



Microscopic description of the rock. — Two well-defined phases 

 of the rock are noticeable: (1) porphyritic, (2) non-porphyritic. The 

 porphyritic peridotite contains abundant phenocrysts of olivine, with 

 a subordinate amount of biotite. The olivine phenocrysts are usually 



1 Loc. cit. 



