GEOLOGY OF THE NORTHERN ADIRONDACK REGION 395 



The Champlain eruptives of this period have received detailed 

 description from Kemp.^ They are more abundant in Vermont 

 than on the New York side of the lake, and on that side seem 

 mostly confined to Essex county, and to the near vicinity of the 

 lake shore. They extend into Clinton county however, in which 

 six small dikes belonging to this group have been found, are still 

 more abundant in northern Vermont and extend thence north- 

 ward into Canada. The Adirondack region seems to have been 

 on the outer border of the region affected by the igneous activity. 



Two contrasted groups of rock are present, the one light col- 

 ored and acid, the other black and basic. The former are classed 

 as bostonites by Kemp, the latter as camptonites, monchiquites or 

 fourchites, according to their mineralogic character. As in the 

 case of the older, Precambric dikes, the acid rocks seem less 

 numerous than the basic and with a more restricted distribution, 

 though these differences are in much less noticeable degree than 

 in the earlier case. 



Trachytes (hostonites) . In New York State rocks of this group 

 seem confined to Essex county, at least none have been discovered 

 elsewhere. Kemp describes them as of prevailing light color, 

 cream}' or brownish white usually, but sometimes a light choco- 

 late; of rough and granular feel and a fracture like that of 

 trachyte. Phenocrysts , are not numerous in general and are 

 nearh^ always of feldspar, quartz having been noted but rarely. 

 The ground-mass is constituted of minute feldspar laths with 

 usually well marked fiow structure. Between the laths small 

 particles of interstitial quartz are sometimes to be detected. The 

 feldspar is both orthoclase and anorthoclase, little or no plagio- 

 clase being present. A considerable amount of hematite is 

 present in minute, disseminated scales, but aside from the above 

 no certain primary minerals can be made out. Calcite, quartz, 

 kaolin and limonite are the principal materials resulting from 

 alteration, and in general the rocks seem hardly as fresh as the 

 older sj^enite porphyries of Clinton county, which they much 

 resemble. 



At Cannons point, just north of Split rock, Kemp has described 

 a large mass of this rock as a sheet, or laccolite, the exposures 



'U. S. Geol. Sur. Bui. 107. 



