222 N. L. BOW EN 



themselves, and it would be necessary for the surrounding plagio- 

 clase to have contained originally nearly one-half potash feldspar, 

 which it certainly did not. It seems more likely that the potash 

 feldspar, though occurring as definite inclusions, was, nevertheless, 

 formed from the portion which remained liquid last and was intro- 

 duced into the plagioclase by a sort of replacement, the change in 

 the plagioclase aureole being an effect going hand in hand with this 

 replacement. 



With the microcline inclusions some interstitial microcline 

 generally makes its appearance, and this may increase in amount 

 until it becomes an important constituent of the rock. In such a 

 specimen the plagioclase is usually andesine rather than labradorite, 

 though the large blue labradorites typical of the anorthosites often 

 occur as phenocryst-like individuals. The rock is definitely inter- 

 mediate between anorthosite and syenite, though the microcline, in 

 the few slides examined, has not as marked a tendency to be per- 

 thitic as it has in the typical syenite. One sees these intermediate 

 types in some of the exposures about the shores of Lake Placid. 

 There is apparently a transition between some of the rocks mapped 

 as gneiss (syenite-granite) and those mapped as anorthosite in that 

 vicinity. 1 Similar intermediate types are found in the vicinity of 

 Elizabethtown, and as a whole they seem to be closely analogous 

 to the perthitophyres of Volhynia as described by Chrustschoff, 2 

 and to the Norwegian monzonites described by Kolderup. 3 



An interpretation of the structural relations of syenite and anortho- 

 site. — While the anorthosite and syenite are evidently closely 

 related and connected by transitional types, they are usually very 

 distinctive. There is one aspect of their field relations in which 

 they are strongly contrasted and with which the writer was 

 impressed in the field. In the great area of syenite-granite that 

 surrounds the anorthosite core, areas of Grenville are exceedingly 

 numerous. In many of the mapped quadrangles it has been neces- 



1 Map accompanying report by Kemp, " Geology of the Lake Placid Region," 

 N.Y. State Museum Bull. 21, 1898. Since the above was written the writer has been 

 informed by Professor William J. Miller that, while the transitional relation is shown, 

 the syenite also sends dikes into the anorthosite. 



2 Tschermak's Min. Petr. Mitt., 9 (1888), p. 470. 



3 Bergens Museums Aarbog, No. V (1896), p. 86. 



