516 



Report of the State Geologist. 



by Kemp from along the lake* Those have a prevailing light tint, creamy or 

 brownish white or chocolate, and nearly or entirely lack magnetite and ferro- 

 magnesian silicates. The Clinton county dikes, on the contrary, are either 

 red or mottled dark red and black, or purplish grey in color (one is nearly 

 black), and with one exception contain considerable amounts of one or 

 more of the minerals, magnetite, biotite, hornblende and monoclinic pyroxene 

 in the ground-mass, and dike No. 9 even has porphyritic biotite. Some of 

 them have a pseudo-schistose appearance. Most of them have porphyritic 

 orthoclase, and this mineral makes up the main portion of the rock. The 

 ground-mass has a trachytic structure, most marked around the phenocrysts. 

 In these last two respects they agree with the typical bostonites, and they are 

 not necessarily more basic because of the greater prominence of bisilicates, so 

 that they are regarded as bostonites varying somewhat from the type, and 

 seem to represent an intermediate stage between these type bostonites and the 

 " granite" dikes described by Marsters from lake Memphremagog.f 



Professor Kemp,- some time ago, called attention to the fact that the basic- 

 dikes are more numerous in the Pre-Cambrian rocks than in the Palaeozoic 

 series, % and further, in discussing the relations of diabase and camptonite, to 

 the confining of the diabases to the former, and of the typical camptonites to 

 the latter rocks. Many of the diabases of the region, however, grade strongly 

 toward camptonites and augite-camptonites, and in two instances of this sort 

 in Clinton county, the dikes are very narrow and seem almost certainly to be 

 ofl'-shoots from large dikes of typical diabase near by, recalling the camptonite 

 apophysae from a laccolite of diabase described by Brogger. 



The diabase dikes are found abundantly throughout Clinton county 

 wherever the Pre-Cambrian rocks are exposed. The camptonites, monchi- 

 quites and fourchites seem to decrease in number north of Port Kent, and 

 north of Plattsburgh are wholly lacking, in so far as negative evidence can be 

 depended upon. This peculiarity of distribution, coupled with the fact that 

 all along the northern line of contact between the Potsdam and the older 

 rocks in the county the diabase dikes are found numerously on one side of 

 the line and not at all on the other, has led the writer to elsewhere express 

 the opinion that there were two periods of dike formation in this vicinity,§ 

 one preceding and one following the deposition of the Palaeozoic rocks of 

 Series IV. The diabase dikes belong to the earlier, and the bostonites, 



* These differences have been noted by A. S. Eakle, who described the first bostonite known in the county. American 

 Geologist, July, 1H93, pp 32 and 33 



t V. F. Marslers. American Geologist, Vol. XVI., pp. 26-29, 

 t Bulletin 107, United States Geological Survey, p. 27. 



5 H. P. Cashing, Transactions New York Academy of Sciences, Vol. XV., pp. 248-252. 



