Il8 NEW YORK STATE MUSEUM 



a mile northwest of Shingle bay of Lower Saranac lake. Several 

 others are located on the southern boundary of the petrological 

 map. One geologist 1 suggests that the anorthosite, which is classi- 

 fied' as an individual of the gabbro family, is the solidified segre- 

 gation or " scum " of lighter substances forming the upper layers 

 of the molten rock. This top portion had more silica and the min- 

 erals crystallizing from it as it congealed were largely feldspars, 

 while the lower layers received the heavier and darker colored 

 compounds richer in iron and magnesium as they settled down by 

 virtue of their greater mass or gravity. 2 Following the solidification 

 of the anorthosite, this dark, lower mass welled up as " normal " 

 gabbro, completing the series. 



Granite 



Taking the Adirondacks as a whole, there appear to be two 

 granites of different ages. In either case the darker minerals are 

 found to have been squeezed into parallel lines by subsequent heat 

 and pressure, giving rise to gneisses. The field worker is often 

 puzzled to know where to place a granite gneiss in the scale of 

 geologic time. In the first place it may be recrystallized or meta- 

 morphosed sedimentary rocks, such as shaly sandstones or sandy 

 limestones of the Grenville group. Their origin is probable if one 

 finds a great variety of such granite gneisses in a comparatively 

 limited area, for this condition suggests their formation from the 

 many different kinds of rocks that composed the Grenville series. 

 If they are surely of igneous origin, the problem is still difficult; 

 they may be (i) older than the Grenville, or (2) later than the 

 Grenville series but earlier than the anorthosite-syenite-granite- 

 gabbro intmsives, or (3) members of the last-named series itself. . 



It may be well to state that geologists have found no positive 

 evidence in the Adirondacks of the class of granites first above 

 mentioned, that is, those prior to the Grenville series. Theoret- 

 ically, they may occur but they have not yet been identified. There 

 is, however, certainty of the identification of the other two different 

 granite gneisses, 3 namely, those that are members of the massive 

 intrusive group and those that preceded them. 



1 R. A. Daly, " Igneous Rocks and Their Origin," p. 337. 



2 More recently, Dr N. L. Bowen suggests a new interpretation. Jour. Geol., 

 XXV, No. 3, p. 209-43. 



3 H. P. Cushing, Amer. Jour. Sci., ser. V, 39:288-94, 1915; N. Y. State Mus. 

 Bui. 145, p. 46-47, 177-80; N. Y. vState Mus. Bui. 169, p. 21-26. Recently the 

 writer has identified them in Paradox lake, Bolton and Whitehall quadrangles # 



