376 PRINCIPLES OF STRATIGRAPHY 



older rock. Here the fauna is richest, and the reefs often rest di- 

 rectly upon the eroded old land. Over the floor of the sea, however, 

 underlain by the black muds, conditions of existence were less 

 favorable, probably because the gases resulting from the decay of 

 the vast number of fishes in these muds were constantly rising, and 

 so the fauna is much poorer as well as more or less dwarfed in 

 species and in individuals. 



With the persistence of the aridifying winds it required only a 

 second closing of the inlet of the North Russian waters to convert 

 the Zechstein Sea into a salt pan, through the gradual evaporation 

 of the water. It is probable that the connection with the sea was 

 temporarily reestablished at least once during this period, this being 

 indicated by the formation of the upper salt series. With the pro- 

 gressive drying up of this sea, the red continental sands encroached, 

 and eventually covered the entire region of the former Zechstein 

 Sea, forming the Bunter Sandstein, the base of the Trias. Thus 

 actual desert conditions were established over Northwest Europe 

 and continued until the invasion of the Muschelkalk Sea. 



The Siluric Salts of North America. These present a rather 

 different type of deposit. In southern Michigan this formation 

 reaches a thickness of nearly a thousand feet. It includes nine 

 separate salt beds, several of them loo feet thick, while the lowest 

 one is i6o feet thick (Lane-32:pl. xl). The annual rings character- 

 istic of the Stassfurt salt seem to be wholly wanting here, and the 

 same is true for the potash salts. The various beds of salt are 

 separated by layers recorded in the wells as shale or limestone, some 

 of which, however, are most probably gypsum or anhydrite. In cen- 

 tral New York the following succession is shown in descending 

 order : 



Bertie Water Lime — Upper Monroe 60 ft. 



Salina Group: 



Camillus shales and gypsum 250-300 ft. 



Syracuse salt 100-470 ft. 



Vernon red shales 200-525 ft. 



Pittsford shales 41 ft. 



GuELPH dolomite of the Niagaran 



The salt series (Syracuse) is thinner than in Michigan. In the 

 well sunk at Ithaca, New York, it has a thickness of only 470 feet 

 and includes seven salt beds ranging in thickness from 17 to 54 feet, 

 with shale partings ranging from 6 to 82 feet in thickness. Where 

 actually exposed, as in the Livonia and other salt shafts, something 

 akin to the annual rings of Stassfurt is seen, in thin intercalated 

 layers of gypsum and shaly material. These are, however, discon- 



