712 



DYNAMICAL GEOLOGY. 



in the earth's history when the vapors first settled on the cooling crust 

 All the materials that were at all soluble, and that the conflict of hot 

 rocks and hot waters could have then made, were at first present in 

 it. An excess of phosphates and of carbonate of lime continued to 

 characterize it after the Paleozoic era had begun, as is learned from 

 the abundance of Lingulce and other phosphatic shells (pages 59, 593), 

 and the profusion of other shells, and of corals. At present, and since 

 Paleozoic time began, the only chemical deposits abundantly made 

 from the waters, in confined basins where evaporation was possible, 

 appear to have been gypsum and common salt. But, with these, some 

 magnesian minerals have been produced, and also some deposits of 

 borates. Salt deposits are now in progress, in confined salt-water 

 basins, along-side of low seashores. 



In the preceding pages on water as a chemical agent, only the 

 more prominent and obvious of the results have been considered. All 

 the ingredients of mineral springs have done work, in the way both of 

 destruction and of construction. A full discussion of the effects be- 

 longs only to a treatise on chemical geology. 



V. HEAT. 



The effects of heat here considered are those connected with the 

 making and modifying of the earth's rocks, strata, and life, exclusive 

 of the comprehensive changes resulting from the earth's gradual refrig- 

 eration. They include (1) expansion and contraction; (2) fusion, 

 solidification, and attending igneous phenomena ; (3) metamorphism 

 and vein-making, besides chemical depositions and changes. After 

 some observations on (1) the Sources of Heat, these subjects are con- 

 sidered under the following heads : (2) Expansion and Contraction ; 

 (3) Igneous Action and Results ; (4) Metamorphism ; (5) Mineral 

 Veins. 



The effects here referred to are mostly due to heat above the ordi- 

 nary temperature. But some geological changes of the widest influ- 

 ence have been carried forward by simple changes in climate. Hence, 

 all sources of change in temperature, however slight, have a geological 

 interest. 



The chief causes influencing the distribution of heat over the globe 

 require a brief notice before proceeding to the subject of the sources of 

 heat. The means of distribution are chiefly oceanic currents and the 

 winds ; and the distribution of the land of the globe determines partly 

 the heat the air receives, and the directions the currents of the ocean 

 take. These points are illustrated on pages 38 to 44. 



The effect of the Gulf Stream on the Arctic and North European 



