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NEW YORK STATE MUSEUM 



apparently not being sufficiently extensive to permit of cer- 

 tainty in the matter. This is the only known instance in New 

 York where any of these rocks occur in other than the dike form. 



Basic dikes (camptonites. monchiquites and fourchites). Of 

 these basaltic rocks the main mineralogic constituents are a basic 

 feldspar, usually andesin or labradorite, augite, brown horn- 

 blende, olivin and biotite. The camptonites are feldspar augite, 

 or feldspar hornblende rocks ; in the monchiquites and fourchites 

 the feldspar retreats or disappears, thus separating them from 

 the camptonites, and they are distinguished from each other by 

 the presence in the former rock, or the absence in the latter, of 

 olivin. Some glassy base is usually present, specially in the latter 

 two rocks, and they not infrequently contain analcite. 



Camptonites are mostly characterized by the presence of brown, 

 basaltic hornblende in sharply bounded crystals. It is often more 

 or less replaced by augite. up to complete disappearance of the 

 hornblende. Such rocks differ but little from diabases, the differ- 

 ence being a minor, structural one ; in the diabases # the augite 

 formed somewhat later than the feldspar and accommodated 

 itself to the feldspar outlines, instead of presenting its own out- 

 lines ; in the camptonites it formed earlier and is more apt to have 

 its normal outlines. In most cases at least some brown horn- 

 blende is present, and serves to distinguish the two rocks. In 

 many of the dikes there is no augite whatever. Magnetite is the 

 only other mineral uniformly present. Some little glassy base is 

 apt to be at hand also. 



The monchiquites consist of olivin. augite, hornblende, biotite 

 (one or all three of the last named) , and a glassy base. Like the 

 camptonites these are apt to be porphyritic. Analcite is not 

 infrequently present. The fourchites are similar except for the 

 lack of olivin. and consist principally of augite, though with some 

 hornblende or biotite. They are much rarer than the monchi- 

 quites in the Champlain district. A related rock, ouachitite, in 

 which the biotite predominates and augite retreats, has not been 

 so far noted in the district, though biotite is abundant in several 

 of the dikes. 



Age of the Champlain dikes. Kemp was the first to note that 

 these acid and basic dikes of the Champlain region are of the 



