GEOLOGY OF SALT IN NEW TOBK 547 



and varies in thickness from 100 to nearly 500 feet. The second 

 member of the series consists of shale and calcareous rock of a 

 light-green color intermingled with a red shale at its lower part. 

 But little gypsum occurs in this member. The rock is extremely 

 porous, easily penetrated by water and falls to pieces at once 

 on exposure to the air. The third or gypseous deposit, which is 

 important commercially on account of its plaster beds, is also the 

 horizon from which the brine springs of Onondaga, Cayuga 

 and Madison counties were supposed by Vanuxem to have 

 been derived. The mass of the deposit consists of rather 

 soft yellowish or brownish shale and slate, both argillaceous 

 and calcareous. It may be called a gypseous marl. It falls 

 to pieces when exposed to the weather, breaking in a series 

 of joints nearly at right angles to each other which give the 

 rock a rhombic cleavage. In the third district the gypsum of 

 this horizon does not often occur in layers or veins, it usually 

 occurs in isolated masses of irregular form. At many points 

 there appear to be two ranges or levels of these plaster beds, as 

 they are called, separated by shale containing hopper-shaped 

 cavities. These cavities, which are from one to ten inches in 

 diameter, are of much interest for they represent the external 

 casts of salt crystals, which were probably formed during the 

 evaporation of the water from the basin in which the Salina 

 deposits were laid down. But few fossils are found in the Salina 

 group, for at the time when the shale and gypsum were deposited 

 the water contained too high a percentage of soluble salts to 

 support animal life. 



The fourth deposit was called the magnesian deposit on account 

 of the assumption that the needle-like cavities were due to the 

 crystallization of sulphate of magnesia. As needle like crystals 

 of sulphate of lime are well known, and as gypsum is abundant 

 in this horizon, it seems more probable that these needle like 

 crystals were crystals of gypsum. 



Prof. James Hall* describes the Salina group as follows : Suc- 

 ceeding the Niagara group is an immense development of shales 

 and marls with shalv limestones including- veins and beds of 

 gypsum. The general color is ashy approaching drab with some 

 portions of dark bluish green. The lower part is of deep red 

 with spots of green. Succeeding this, where protected from 



* Geology of the Fourth District. 



