2^ NEW YORK STATE MUSEUM 



As far as the writer has been able to learn, the geologically 

 intermediate Hamilton shale has, until now, yielded no dikes and 

 the recent discovery of two in this formation at a locality about 

 12 miles southwest of Syracuse and about 40 miles northeasterly 

 from Ithaca is believed to be a matter of interest. 



The dikes in question are exposed on the south wall of the 

 Clintonville ravine at a point approximately 50 feet above the 

 level of the Marietta road. The more western is a fine grained 

 porphyritic rock resembling peridotite. What appear to be ser- 

 pentine grains, produced by the alteration of olivin, protrude 

 from the weathered surface and have the appearance of small 

 pebbles. Another conspicuous feature is furnished by large scales 

 of a bronzy mica. This dike has a uniform width of from 7 to 

 8 inches and is displayed for about 12 feet on the south bank of 

 the ravine. On the north side it is obscured by talus. Its plane 

 is vertical, while its direction is north and south, agreeing in this 

 latter respect with the Ithaca dikes. Wherever examined it 

 presents a very uniform texture, is apparently free from frag- 

 ments of the sedimentary rocks through which it passed, and has 

 produced little contact metamorphism. 



The second dike discovered by the writer lies about 2 feet and 

 4 inches to the east of the first and was not observed until the 

 wall at this point had been cleaned. It has a width of about 8 

 inches. Like the first dike, it is vertical and north and south in 

 direction. It differs, however, from the first dike in being much 

 weathered in places and in containing many shale fragments some 

 of which have a long diameter of 3 inches or more. 



Dr Smith has, at my request, supplied specimens from these 

 interesting occurrences for examination by Prof. C. H. Smyth jr, 

 whose acquaintance w^ith the other dikes on the sedimentaries 

 of central New York is intimate, and the latter has supplied some 

 notes thereupon, essentially as follows : 



They are greatly altered alnoites. The olivin is completely 

 broken down into serpentine, magnetite and, often, carbonates. 

 At the same time, the mica remains surprisingly fresh, as it does 

 in other dikes of the region. Perofskite, so abundant in the 

 Manheim cHke, is much less conspicuous here, while the critical 

 mineral, so far as classification is concerned, melilite, is almost 

 obliterated. One section from each dike, however, shows what 

 I consider quite unmistakable traces of melilite, in one case a 

 good deal of it, and I have no doubt that if the material were 

 entirely fresh, the rock would prove to be quite typical alnoite. 



At first glance, it may appear that the presence or absence 

 of melilite is a trifling matter, since, at most, it probably would 

 be only a minor constituent, the rock consisting chiefly of olivin 

 and mica, and thus, for the most part, resembling, if not belong- 

 ing to, the peridotites. But, really, the question is of more than 

 ordinary interest, for as soon as melilite appears in an olivin rock 



