46 A GEOLOGICAL HISTORY. 



slimey, clayey argillaceous matter, highly charged 

 with oxide of Iron, some of which deposit still 

 remains between and separates each layer. All 

 do or will admit, that this Sandstone, was once 

 grains of sand, like the sand on our beaches, but 

 by time and the natural chemical action which is 

 always going on, it has become indurated and 

 cemented. All bodies that can absorb water, 

 swell, and as they loose it shrink, (on this principle 

 << Wedgeworth's Pyrometer" is made,) by this ex- 

 pansion and contraction the seams were made, and 

 as the clay or argillaceous deposits were not so 

 prone to cement as the silicious, there the seams 

 do still exist ; clay when drying shrinks, as this 

 clay dried, it adhered to the upper and under 

 surfaces, and in time became indurated, and by its 

 shrinking it naturally formed these impressions 

 called " rain marks." If any one will take plastic 

 clay, make it into a soft batter or paste and put 

 some of it between two roof slates, the phenomenon 

 will be partly explained by pulling them apart. 

 Many writers on Geology, do not take into con- 

 sideration, the great chemical, as well as the 

 mechanical changes which are going on ; the 

 influence of heat and its negative cold, magnetism, 

 wetting and drying, solution, attraction, deposition, 

 and cohesion, all have a tendency to alter and 



