STEVENSON— INTERRELATIONS OF FOSSIL FUELS. 45 



The lenticular form of coal seams is as distinct in the Jura and 

 Trias as it is in later periods. It is characteristic of Jurassic coals 

 in Great Britain, France, Austria, Hungary, Siberia and Queensland, 

 as well as of Triassic coals in France, Austria and the United States. 

 Direct reference to this feature is not made in some of the earlier 

 reports as, at the time the studies were made, the bearing which 

 the form of coal seams has upon the problem of their origin was 

 not recognized. But in every area the varying thickness of coal 

 seams is emphasized ; the frequent passage of coal into carbonaceous 

 shale is noted ; the presence of coal seams in some vertical sections 

 and their absence from others attracted the attention of all observers. 

 The lenses may have considerable area but often they are small ; 

 they may be thick or thin. Those of the Tchoulym field of Siberia 

 have small superficial extent, rarely exceeding a few square kilo- 

 meters and they are rarely connected, but their thickness is so great 

 that the Russian geologists speak of the total quantity of coal in this 

 district as "colossal." 



References to contemporary erosion are rare in the reports. 

 Wilkinson has recorded instances of filled channel ways in the 

 Triassic of New South Wales and Hertle has described an in- 

 teresting " horseback " in a Triassic seam near Rehgarten in Austria. 

 The irregularities in the roof of coal seams in the Richmond field, 

 as described by several observers, have much resemblance to " horse- 

 backs," but the mines in which they were seen were abandoned half 

 a century ago, so that one cannot determine whether or not these 

 irregularities are due to trenching of the coal seams. 



Soils of vegetation have been reported from England and the 

 United States, but, if they be present elsewhere as one should think 

 probable, observers have failed to make note of them. In such 

 soils one finds vertical stems of plants, rooted apparently in place 

 of growth but not associated with seams of coal. The Purbeck 

 " dirt beds " of southern England have stumps of conifers and 

 cycads rooted in carbonaceous clay. Mantell states that the conifer 

 stems have lost their bark and have a weatherbeaten surface like 

 that of posts set between tides. They resemble the stumps exposed 

 above the Yahtse gravels, as described by Russell. Stems of the 

 Purbeck conifers were snapped ofif at 3 or 4 feet from the ground 



