r88 



NEW YORK STATE MUSEUM 



spar, or mica, or both, to give it foliation. Though broken at 

 intervals, this ridge is quite persistent for four or live miles, when 

 ii disappears, being, like the gneiasoid schist to the southward, cut 

 out by the groat mass of granite-gneiss that makes up the south- 

 ern end of the island. 



With a single exception to be mentioned below, the writer has 

 seen now here else in the crystalline area a body of quartzite so 

 large and so pure as this, and it is of particular interest as afford- 

 ing an unquestionable example of a pre-Cambrian rock of sedi- 

 mentary origin. In the highly metamorphic series, estimates of 

 thickness must always be quite hypothetical, but in this in 

 stance, 500 feet would seem to be a Aery conservative estimate 

 of the thickness of the quartzite; The strike of the belt and of 

 its foliation is n 45° e and the foliation dip 80° n. 



Though these members of the schist formation make up the 

 great part of this end of the island, the granite gneiss is not want- 

 ing, but appears in scattered areas of limited extent. There is 

 abundant evidence that these masses of gneiss are younger than, 

 and intrusive in, the schist. Masses of gneiss of considerable size.' 

 break through the schist and quartzite in the most irregular 

 fashion, while the latter rocks are penetrated by granitic dikes 

 which range from fractions of an inch to yards in width. These 

 dikes are usually, though not always, more massive than the bulk 

 of the gneiss, and often coarser grained, even pegmatite, but that 

 they are offshoots of the gneiss is evident. The same structural 

 differences have been noted in similar instances described pre- 

 viously. 1 



Farther and even more striking evidence of the relation of the 

 two formations is afforded by great quantities of inclusions of 

 schist and quartzite in the gneiss. These vary greatly in size and 

 shape, though, particularly in the case of the well foliated and 

 banded schists, they are apt to be elongated in the direction of 

 foliation. In some cases, indeed. I hey appear as dark bands in 

 the gneiss, looking at first sight like broken dikes. Occasionally 



Crystalline rocks of St Lawrence county, 15th an. rep't N. Y. state 

 geologist. 1895. p. 491. 



