392 SILURIAN PERIOD 



of shales in general, but the fewness of its fossils points to deposition 

 under conditions unfavorable for life. The salt is widely distrib- 

 uted. In New York alone it occurs at many points within an area 

 of 9,000 to 10,000 square miles. Single beds of it are locally 40 to 

 80 feet thick. In places, several beds occur one above another, 

 interstratified with other kinds of rock, and their aggregate thickness 

 reaches as much as 100 feet in places. Near Cleveland, four salt 

 beds, 50 feet and less in thickness, are interstratified with 500 feet 

 of shales. 



The salt beds seem to imply the existence of great lagoons or 

 inclosed seas. Deposits of gypsum are made under about the same 

 conditions as salt beds. Had the climate of this region been as moist 

 as now, lagoons could not have been abnormally saline. Occasional 

 incursions of the sea, bringing in new supplies of salt water, followed 

 by periods when the lagoons were cut off from the sea, and when they 

 suffered rapid evaporation, would seem to meet the conditions de- 

 manded for the salt. So also would a slight continuous connection 

 with the sea, such that the inflow of sea- water into the basin was less 

 than the excess of evaporation over precipitation. A bed of salt 40 

 feet thick implies the evaporation of some 3,000 feet of normal sea- 

 water. Much of the commercial salt which comes from New York 

 is derived from the waters of salt wells. 



The limestone of the Salina contains few fossils, and has been 

 thought to be a chemical precipitate. The relations of limestones, 

 shales, and salt beds are such as to indicate that the sites where these 

 several sorts of rock were formed shifted from time to time, as if by 

 gentle crustal warping. 



Above the Salina proper of New York, there is a thin (150 feet 

 maximum) series of limestones (p. 388), more widespread than the 

 Salina. Its equivalent extends westward through Ohio to Indiana 

 and Wisconsin. Both its distribution and its character show that 

 the eastern interior was more generally submerged than during the 

 deposition of the salt-bearing series which preceded. 



The Helderberg formation, formerly regarded as a part of the 

 Silurian system, is here classed with the Devonian. 



Summary. As in the preceding systems of the Paleozoic, the 

 greatest thicknesses of Silurian strata (estimated at about 5,000 feet, 

 maximum) are in the Appalachian region. Over the interior, the 

 system is measured by hundreds of feet, rather than by thousands. 

 In keeping with these variations of thickness, the system is largely 



