Feather slonhaugli^s Geological Report. 



Ill 



The sandstones are veiy fissile, owing to the disposition of 

 the mica ; they consist of rounded granular quartz, from 

 massesof which their constituent parts may have been detached 

 with the mica. In this countiy there are numerous thick beds 

 of sandstone formed with rounded grains of quartz in the 

 upper part of the carboniferous limestone formation as w^ell as 

 the millstone grit, which have no mica. The sandstones with 

 rnica, here spoken of, are much less coherent than the mica- 

 ceous sandstones in connexion with the primary rocks, of 

 which there are some fine examples up Rock creek, in the 

 District of Columbia. 



These fissile sandstones frequently contain stems and frag- 

 ments of terrestrial plants, and are often separated by beds of 

 limestone containing: marine s hells. The abrupt changes of 

 these mineral strata, and their organic contents, often without 

 admixture, show that they have been deposited not at distinct 

 periods alone, but under circumstances widely different. The 

 strata appear at one time to have been covered with calca- 

 reous salt waters, which subsequently became dry land, and 

 afterwards received argillaceous deposites of mechanical origin, 

 brought by fresh water, in the manner alluded to when the 

 wealden group was treated of. In some of these shales, the 

 remains of fossil unios are found associated with the plants, 

 showing that the same state of things existed in the muds of 

 the estuaries and rivers of ancient geological periods, which 

 we are constantly observing on this continent under the pres- 

 ent order of nature. These unios are found in great abun- 

 dance in the Jarrow colliery, in the Newcastle district, many 

 of them lying with the valves gaping open, and proving con- 

 clusively that the bed where they are now found was once 

 the surface of the earth, though now many hundred feet below 

 it.^ This bed is only one of a number similarly situated, and 



* Similar observations have been made here. Dr. Hildreth, p. G9, 70, observes 

 that fossil unios, rnelania, and lymnea, all fresh-water genera, are found "in a 

 bed of dark carbonaceous clay," at a level many feet below the coal. 



