GEOLOGY OF PART OF SOUTH DURHAM. 173 



very unsafe conclusion to suppose that the vegetable substance 

 of such plants had much to do in forming the seam. But on the 

 other hand, it is evident that a seam of coal and its roof repre- 

 sents two separate conditions of things — one favourable to the 

 growth and accumulation of the vegetable matter forming the 

 coal, and the other favourable to the accumulation of the earthy 

 matter forming its roof; and as roof-matter is not a growth, but 

 sedimentary materials drifted from a distance, it is possible, to 

 say the least, that many of the remains of plants imbedded in 

 the roof may have been transported along with the sedimentary 

 materials, and be in a measure foreign to their immediate area 

 of occurrence. 



The species have mainly been determined from the "Fossil 

 Flora" of Lindley and Hutton, and probably with these authors 

 we have got different portions or growths of the same plants as 

 distinct species. But we do not attempt to treat this branch of 

 the subject, our researches having had relation to the distribu- 

 tion of the fossils rather than to a critical examination of their 

 affinities. 



The majority of the species in this list have been previously 

 noticed in the Durham and Northumberland coal-field, in the 

 middle and upper beds of the series. But only twelve species 

 have hitherto been recorded from horizons so low as the Brock- 

 well and Five Quarter seams. Nine of the twelve have occurred 

 to us among the seventy-three of our list. Fifty-six species 

 occur in the Brockwell, and fifty in the Five Quarter and Har- 

 vey. Thirty-two species, including all the common and charac- 

 teristic forms, are found in both Brockwell and Five Quarter. 



Species and individuals are both very irregularly distributed. 

 The most common fossils are Sigillariae and Galamites, both of 

 which occasionally occur alone with great individual profusion, 

 and often intermixed with ferns, Lepidodendra, and other vege- 

 table fossils. 



With regard to Sigillaria, we well remember a sight that we 

 saw in the Brockwell seam, at Howdon Colliery, where the roof 

 was one entire mass of specimens of this genus. Among them 

 were several species or varieties which were stretched overhead 



