COALS AND COAL SHALES 513 



The cliaracters of the Ferns and the Sphenophyllacese of the 

 Carbonic likewise point to moist tropical conditions during that 

 period, and since the coal deposits resulting from the growth of 

 these plants are so widely distributed in regions which to-day have 

 a temperate climate, it is apparent that conditions now characteristic 

 of the equatorial region of the world were during the Carbonic 

 period more widely distributed. In Chapter II a possible cause 

 for the increase in temperature was discussed, this being the greater 

 supply of COo in the atmosphere which would retain the heat and 

 so raise the temperature of the entire earth's surface. The removal 

 of this atmospheric constituent and the storage of the carbon in 

 the tissues of plants would bring about a progressive lowering of 

 the temperature, and this refrigeration of the climate would bring 

 on the glacial conditions known to have existed in Permic time. It 

 is a notable fact that the coal-forming period was preceded by a 

 period of wide and extensive submergence and the building of lime- 

 stones in all parts of the world. This would result in setting free 

 much COo, and hence would supply the conditions favorable to the 

 development of the tropical coal swamp on the emergence of the 

 land.* 



The coals formed during the J\Iesozoic periods have probably a 

 history similar to that of the Carbonic coals, though seashore con- 

 ditions and those of lagoons and estuaries may have been more 

 common. The Tertiary coals, especially those of northern Europe, 

 appear to have been formed in swamps closely similar to our mod- 

 ern cypress swamps of southern North America. , Many of the 

 species found in the brown-coal deposits of North Germany, such as 

 the Magnolias, Liquidambar, Sassafras, Catalpa, Swamp Cypress, 

 etc., are still living in these swamps, though they have become ex- 

 tinct in Europe. The remarkable depth of the brown-coal deposits 

 of North Germany, reaching north of Cologne nearly 100 meters, 

 gives some indication of the length of time required to accumulate 

 such a mass of vegetal material by successive periods of growth and 

 decay. As in the case of the Carbonic deposits, the Tertiary coal 

 period was preceded by extensive limestone deposits and succeeded 

 by a period of gradual refrigeration which culminated in the Pleis- 

 tocenic glaciation. 



Black Soil and Shales of Humulithic Origin. 



Long-continued growth of vegetation produces in some regions 

 a thick accumulation of a dark loam, such as is seen in the black 



* See also p. 90, and the reference to Koken's work in Chapter XXIII. 



