26 NEW YORK STATE MUSEUM 



and the writer has elsewhere shown that the appearance is due 

 to a secondary calcite cement deposited between the sand grains, 

 so that they become as it were incorporated in calcite crystals 

 whose cleavage gives the characteristic appearance to the broken 

 rocks.^ 



Many of the beds of the formation, in the Little Falls district, 

 are full of small, drusy cavities which are in general coated with 

 minute dolomite crystals, sometimes with calcite as well. In these 

 cavities are often One or more quartz crystals, generally small and 

 water clear, though sometimes of quite large size, the latter usually 

 full of inclusions. These crystals have long been known and have 

 made the district a famous one to the mineralogist. They are 

 locally known as diamonds and have given the name to Diamond 

 hill, where they are very abundant, as they are also about Middle- 

 ville. In addition, the cavities often contain much of a black, 

 carbonaceous material, sometimes nearly filling the cavity, some- 

 times as films on which the dolomite crystals rest, sometimes 

 running into cracks of the rock, and sometimes occurring as 

 inclusions in the quartz crystals in a finely divided state. This 

 material has heretofore been called anthracite, behaves precisely 

 like that substance when heated and must have the same approxi- 

 mate chemical composition, though with somewhat different physi- 

 cal properties. From the standpoint of origin it would seem to be 

 certainly an asphalt derivative. It was the first substance to form 

 in the drusy cavities and was followed by the dolomite crystallizar 

 tion,; though the two seem to have overlapped somewhat. Both 

 the quartz and the calcite were formed after the dolomite. The 

 writer observed no instance of a cavity in which quartz and calcite 

 were both present ; so that it can not be stated which of the two 

 was formed first. 



Near or at the summit of the Beekmantown formation, is a very 

 cherty layer, becoming locally a pure mass of chert, which is 

 sometimes red in color. This cherty layer often has a mineralized 

 appearance, due to abundant, small, bluish green spots which 

 have some resemblance to green copper carbonate (malachite). 



^N. Y. State Geol. 16th An. Rep't. 1896. p.l9. 



