Field-work in a portion of the Adirondack region during the past two years has disclosed the rather widespread occurrence of rocks which resemble some phases of thè anorthosites, and were classed with them until inspection of thin-sections showed their quite different nature. Similar rocks prove to be of frequent occurrence in the district, and to have an extent and importance not heretofore recognized. They also possess considerable intrinsic interest, so that some preliminary notice of them would seem to be justified in advance of a thorough investigation of their field relations. These rocks are widely and quite typically exposed in the vicinity of Loon lake, in Franklin county, New York, and many of the exposures are easily accessible; hence their selection for descriptive purposes. The rocks are referred to the augite-syenites.
When fresh these rocks are of a grayish green color, which quickly changes to a more . . .