REPORT OF THE DIRECTOR AND STATE GEOLOGIST 1900 r41 



these are exceptional. There are no such great masses of gab- 

 bro as Kemp has shown to exist in Essex county, involved with 

 the anorthosite.^ 



OBSERVATIONS ALONG THE SOUTHWEST AXORTHOSITE BOUNDARY 



Within the past four years Prof. Kemp, Prof. Smyth and I have 

 all observed considerable areas of rocks, in our respective dis- 

 tricts in the Adirondack region, which much resemble the 

 anorthosites in some of their phases and were at first classed with 

 them. These rocks have great importance in the region, and their 

 true nature was realized by all at about the same time and 

 independently.^ Like the anorthosites, they are highly feld- 

 spathic rocks, but differ from them in nature of the feldspar, so 

 that they must be classed with the syenites rather than as mem- 

 bers of the gabbro family. All of us agree that these syenites 

 are younger than the Grenville rocks. In Smyth's district 

 anorthosites are practically absent, so that opportunity for deter- 

 mining the relative ages of the two rocks is not forthcoming there. 

 Kemp seems disposed to class them with the main mass of the 

 gneisses and hence as older than the anorthosite. The writer 

 noted the intermediate rocks which have been described in the 

 previous pages and was led to believe that the two rocks were 

 closely related in age, were derivatives from the same great 

 igneous magma, and even in part derived from differentiation of 

 the anorthosite itself. In the hope of settling the question, con- 

 siderable time has been given to observationsi along the southwest 

 anorthosite boundary, as it immediately adjoins a large syenite 

 area in that direction. Exposures are however few near the 

 boundary and have not suiSced to give a definite answer to the 

 question at large, though clearly bringing out the fact that some 

 of the syenite at least is closely related to and slightly younger 



^See map fronting p. 398, 19th an. rep't U. S. geol. sur. 1899. pt 3. 

 » Smyth, C. H. jr. Geol. soc. Am. Bui. 6 : 271-82. 



X. Y. state geol. 17th an. rep't. 1897. p. 471-86. 



Kemp, J. F. N. Y. state geol. 17th an. rep't. 1897. p. 507, 512, 515. 



Gushing, H. P. Geol. soc. Am. Bui. 10 : 177-92. 



N. Y. state geol. 18th an. rep't. 1888. p. 105-9. 



