STEVENSON— INTERRELATIONS OF THE FOSSIL FUELS. 191 



fossils and occasionally a bed with typical marine forms. At most 

 localities the bedding is irregular and the rocks seem, in many cases, 

 to be composed of dove-tailing lenses ; lenses of fine clay occur 

 frequently. Sandstones and sands are cross-bedded in great areas ; 

 ripple-markings and mud-cracks have been reported from many 

 places. Conditions in the western United States appear to indicate 

 that these deposits were made on plains, where the sands drifted 

 and where the water was collected in shallow pond-like areas. The 

 gravels in many cases indicate filled channel-ways. 



These intervening rocks frequently contain drifted materials. 

 Leaves and stems of upland vegetation are found in the sands of 

 Siberia ; Collier saw cones of Picca, bones of mammals, land and 

 freshwater shells in beds overlying Pliocene lignite on the Yukon ; 

 Grand'Eury observed wood, roots and spots of lignite at Budweis; 

 Colenso in Wales described a deposit very like that of Alaska; the 

 sections in western North America usually contain reference to these 

 drifted plant remains. But not all the plant remains were trans- 

 ported ; at many places they mark old soils of vegetation, surfaces 

 where plants grew, but not long enough for accumulation of coal. 

 There are many references to these in preceding pages and only one 

 need be added. Darwin,-^- in a publication later than his " Re- 

 searches," gave a detailed description of the petrified forest seen on 

 the Uspallata range of Chili. The stumps, exposed in a small area, 

 were in green and brown sandstone, which had been removed by ero- 

 sion so as to show the erect stems in place. Fifty-two stumps were 

 examined, projecting 3 to 5 feet — in one case 7 feet — from the sur- 

 face. Whether or not they were rooted, he could not determine, as 

 the lower part in every case was still buried in the rock. The dip 

 was 25 degrees and the stems were vertical to the bedding. It was 

 suggested to him that the trees might have been transported, but 

 that explanation seemed insufficient ; it might suffice for a single 

 stump but not for a clump of 52 trees, which belonged to an inland 

 coniferous flora, not to a coast vegetation. The conditions convinced 

 him that erosion had laid bare only a small part of the forest. 



Occasionally, lake marls, interrupted by beds of brown coal, 



-'''- C. Darwin, " Geological Observations in South America," London, 

 1846, pp. 202. 



