THE TERTIARY FORMATIONS OF VIRGINIA. 357 



riileiitand chalk marls, are found in extensive beds on Curratoman river 

 and Carter's creek, west of the termination of the higher level of the 

 neck. 



In general, the upper beds of the meiocene, in this district, are des- 

 titute of fossils, though full of their casts and impressions. These 

 strata, consisting, for the most part, of light coloured sandy clays, fre- 

 quently of great depth, are distinguished by a sulphurous smell, and 

 an acid and somewhat styptic flavour. They rarely contain any con- 

 siderable amount of carbonate of lime, presenting, in its stead, variable 

 and sometimes valuable proportions of the sulphate, together with sul- 

 phate of iron, sulphate of alumina, free sulphuric acid, sulphur, and 

 sometimes even an appreciable quantity of sulphate of magnesia. 



The acidity of these clays is often sufficient to make a pungent im- 

 pression on the tongue, and their sulphur is distinctly recognized by the 

 characteristic odour they exhale, especially when gently warmed. In 

 many localities the gypsum occurs in crystals of sufficient magnitude 

 to be readily separated by the fingers, and sometimes even in the at- 

 tractive form of transparent sellenite ; but its more usual condition is 

 that of delicate silken crystals, distributed through the mass, and visi- 

 ble only upon close and attentive inspection. The sulphates of iron 

 and alumina are occasionally observed in the form of an efflorescence, 

 upon the surface of the strata, and the gypsum likewise presents itself, 

 under similar circumstances, as a white incrustation. Minute, silvery 

 scales of mica are met with in nearly every stratum, but abound most 

 in those of a bluish or greenish tinge. 



The fossil impressions contained in these beds are, in general, beau- 

 tifully distinct, and appertain to all the species of shells which are found 

 in perfect condition in the subjacent strata. In some cases the over- 

 lying band of iron stone is not less richly fraught with them than the 

 layers beneath, and from its hardness and insolubility, has preserved 

 the most delicate markings of the shells in all their original sharpness. 

 In many localities the impressions of the fossils in the clay or sand are 

 beautifully bronzed by a thin film of oxide of iron, which has taken 

 the place of the material of the shell ; but in others a vacancy seems to 

 exist in the space originally occupied by the calcareous matter, so that 



