318 THE STRUCTURE AND SUCCESSION AT 



carbon dioxide derived from decaying vegetation and deposited 

 subaerially, by evaporation of water or by loss of carbon dioxide, 

 or it may have bsen formed under the sea from shells. The 

 presence of land or marine fossils would decide the question 

 one way or the other. I hive failed to find them, but Mr. 

 Brown and Mr. Robb report marine fossils. This makes it an 

 ordinary shell deposit. Re-elevation at last brought an end to 

 the formation of limestone, and renewed the supply of "sand 

 which in turn was succeeded by clay as before. After a pro- 

 longed period of clay deposition the waters at length became 

 clear, and gypsum was deposited. 



Gypsum. — Two theories have commonly been advanced as 

 to the origin of gypsum beds : — (1) formation by the action of 

 -sulphuric acid on limestone ; (2) precipitation from water solu- 

 tions by other s.dts, or by partial evaporation. In the case 

 before us either method could have operated. The rocks both 

 above and belovv are limestones containing iron. The most 

 fundamental compound of iron occurring in nature is ferric 

 sulphide, iron pyrites. By oxidation or through organic 

 influence, this iron is often changed to oxide or carbonate, and 

 finally in a state of solution impregnates other rocks. The 

 sulphur goes to form sulphuric acid. The presence of iron in 

 these rocks points to a pi'obable presence, then, of sulphuric 

 acid. It being supplied, and the limestone already at hand for 

 it to attack, gypsum could result just as we find it. 



On the other hand, since it is so regularly bedded in material 

 evidently accumulated under water in a territory of alternate 

 elevation and subsidence, there is no valid objection to the 

 precipitation theory. In some parts of the world, beds are 

 undoubtedly of this origin, where they alternate with rock salt. 

 The absence of an overlying salt bed here can be readily 

 accounted for, from the evidence of seashore and marine deposi- 

 tion where water could not be sufficiently concentrated to pre- 

 cipitate its common salt. It is possible, moreover, that the land 



