GLAUCOXITE IN THE POTSDAM. 95 



The many peculiar features in the structure and variation of the Pots- 

 darn and its abundant fossils scarcely exceed in interest the discovery of 

 its abundant greensand or glauconite grains. In almost every exposure 

 of the sandstone where careful examinations were made the grains were 

 found, and in some places they exist in such quantities as to give a green 

 color to the rock. Though their presence is evidently a persistent feature 

 of the formation, the reader who may wish to repeat our observations is 

 referred especially to Castle Creek, to lower French Creek, and to the lower 

 cafion of Eapid Creek. Usually the same portions of the sandstone 

 are both glauconitic and calcareous, but the association is not invariable. 

 Ordinarily the glauconitic sandstone is merely speckled with the green 

 grains, but in some localities the glauconite composes the larger mass of the 

 rock. The calcareous matter is often not evident until revealed by the touch 

 of an acid. When the glauconitic sand-rock is treated with acids the lime 

 matter is dissolved, and the rock readily crumbles into loose grains of sand 

 and glauconite. The glauconitic grains are rarely over one-tenth of an 

 inch in their larger diameters, and the majority are not more than half that 

 size, and from this they range to almost microscopic sand. They are almost 

 always flattened, and under a low-power magnifying glass appear as disks 

 or flat flakes of irregular shape, round, oval, reniform, etc., but always 

 smooth and nicely rounded as though water- worn. Their color is a dark 

 olive green, sometimes nearly black, and when crushed the powder has a 

 bright olive-green color. Their hardness is between 4 and 5 of the scale. 

 Under a microscope of low power they appear perfectly regular in outline, 

 their surfaces smooth but somewhat pitted, and their color dark olive green, 

 but over the edges the green color is lighter. They are oqaque except on 

 very thin edges, and there translucent or semi-translucent. They are only 

 slightly acted upon by strong hydrochloric acid. Before the blow-pipe 

 they fuse to a black magnetic mass, and in salt of phosphoric lead dis- 

 solve with difficulty, leaving skeletons of silica and giving in the bead the 

 usual iron reaction. In these blow-pipe characters they resemble perfectly 

 the greensand grains from the Cretaceous of New Jersey. The latter, 

 however, are not so hard, and are less smooth and rounded, having botry- 

 oidal forms as though concretionary in character. 



