Cushing — Geology of Clinton County. 



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slates must he well up in the Trenton. It will be impossible without pro- 

 longed study to arrive at a just estimate of their thickness here. The apparent 

 thickness on Cumberland Head is very great ; but there are also evidences 

 there of much disturbance, and it is strongly probable that there is repetition. 

 In addition to the evidence afforded by the slaty cleavage, one or two faults 

 are plainly visible in the slates, and the topography strongly suggests others. 

 Yet, taking all these things into consideration, 200 feet would be a very 

 modest estimate of the thickness here displayed, and double, or even treble 

 that amount would most likely be much nearer the true figure. The entire 

 Trenton in this vicinity, therefore, must be very thick. The writer had the 

 pleasure of conducting Mr. van Ingen to Cumberland Head, and to the ex- 

 posures of these same slaty rocks in Beekmantown, and since the above was 

 written a letter has been received from him giving the results of his study of 

 their fauna. He pronounces the horizon to be high up in the Trenton, and 

 with features in common with the " Quebec group " of Canada, so that it may 

 turn out to be more nearly' allied to the Utica slate than to the Trenton 

 limestone ; that is, the Utica as represented in Canada. 



Utica date. On Stony Point, in Champlain township, is an exposure of 

 black, ringing, much-jointed calcareous slates, the joints largely tilled with 

 crystalline calcite, which carry the ordinary fauna of the New York Utica 

 slate, Triarthrus Becki, (Jlimacogra/ptus hicornis and other graptolites, and 

 Mndocerm. With the exception of this single exposure, shut in on the east 

 and north by the lake, and on the west and south by a marsh along a line of 

 fault, no rocks known to be later than the Trenton limestone occur in the 

 county. The Utica slate is also exposed on the east side of Isle la Motte. 



Merits V. The various types of dike rocks occurring in the lake 

 Champlain district have been described at length by Kemp and Marsters,* 

 and would need little mention here were it not for the fact that their 

 study in Clinton county has emphasized certain facts regarding their dis- 

 tribution, which are not so apparent in Essex county and along the lake. 



The dikes are readily separable into two classes ; black, basic dikes of 

 the varieties known as diabases, camptonites, monckiquites and fourchites, and 

 lighter-colored acidic dikes, closely approximating the same type, and known 

 as bostonites, the term being applied to the dike-form of feldspar porphyries 

 or trachytes. 



The acidic dikes known in Clinton county (twelve have so far been 

 found) differ considerably from the typical bostonites us found and described 



'Bulletin 107, •United States Geological Survey. 



