GEOLOGY OP FRANKLIN COUNTY 99 



5tion may be traced within a few feet into the hornblende gneiss, 

 which has been produced by the metamorphism of the latter. An 

 ■excellent locality for such exposures is the west shore of Upper 

 Chateaugay lake at the point where it narrows into the outlet. 

 Another is a cut on the Northern New York railroad 2 miles south 

 ■of St Regis Falls. 



It is however the exception when the hornblende gneisses can be 

 traced to a connection with the gabbros. The question at once 

 arises whether there are hornblende gneisses of two distinct ages 

 or they are all to be considered as derived from the gabbro. This 

 question can not be answered as yet, though the writer rather 

 inclines to the first alternative. Such gneisses are excellently 

 «hown around Dickinson Center, interbanded with the orthoclase 

 gneisses in such fashion as to produce a strong impression that 

 the latter represent granitic intrusions into the former, the whole 

 drawn out and foliated by the metamorphism. 



Later eruptives 



To a considerable extent the Precambrian rocks exposed in 

 Franklin county are younger than the Grenville rocks, and 

 younger than most, at least, of the doubtful gneisses as well. 

 These rocks are all of igneous origin, occurring as immense in- 

 trusive masses or as dikes, which show eruptive contacts against, 

 and also include masses of the Grenville rocks and of the gneisses. 

 Most of them have been metamorphosed and given a foliated struc- 

 ture, so that they are gneissoid, but frequently massive cores re- 

 main, into which the gneisses can be traced by a gradual trans- 

 ition. Quite similar gneisses also occur with no apparent massive 

 •core, and the recognition of such and their separation from similar 

 phases of the earlier gneisses, as well as the accurate mapping of 

 the boundaries of the intrusions themselves is often a matter of 

 the greatest difficulty. These eruptives comprise rocks of the 

 gabbro, syenite and granite families. 



Anorthosites. The name anorthosite was given by F. D. Adams 

 to eruptive rocks which occur widely in eastern Canada, are re- 

 lated to the gabbros and consist almost wholly of labradorite 



