ON A BASIC ROCK DERIVED FROM GRANITE. 



While studying the hematite deposits of St. Lawrence and 

 Jefferson counties, New York, under the direction of Dr. James 

 Hall, State Geologist, the writer had occasion to examine some 

 interesting rocks associated with the ores, which are worthy of 

 note. 



A brief description of the rocks is given in the report on the 

 region presented to Dr. Hall, in which especial attention is paid 

 to their relation to the problem of the origin of the iron ores. 

 The aim of the present paper is to supplement that report with 

 a more complete account of one phase of these rocks, occurring 

 at the Old Sterling mine, in Antwerp, Jefferson county. 



Mode of occurrence of the rock. — The mine consists of a large 

 open pit, together with considerable underground workings. 

 The surface rock is Potsdam sandstone, beneath which the ore, 

 a red hematite, occurs in large irregular masses, intimately asso- 

 ciated with a rock of entirely different character, which is the 

 subject of this paper. The contact between this rock and the 

 ore is extremely irregular, as is well shown in the open pit where 

 the ore has been removed leaving the rock in projecting knobs, 

 ridges, and walls, with intervening pockets and hollows. In the 

 underground workings it is not uncommon for the ore to be sud- 

 denly cut off by the rock and to appear again after passing 

 through a greater or less thickness of it. The contact is evidently 

 irruptive and is precisely like the contacts of granite and limestone 

 which are common in the region, although the rocks involved have 

 a very different appearance. Emmons, 1 who has given the only 

 detailed account of the ore and associated rock, considered 

 them both igneous, and called the rock serpentine, a name 

 which has clung to it ever since. The facts adduced by Emmons 



'Geology of New York. Second District, p. 97. 



667 



