208 Mr. Wood's Account of some Fossil Stems of Trees. 



Dyke which ranges north and south, when the dip changes and becomes 

 due west. Near the great Dyke, the immediate cover of the high Main 

 Coal is Sandstone, or " Post," compact, varying in thickness from 12 to 

 15 fathoms, and called the "Main Post." This stratum of Sandstone 

 gradually diminishes in thickness from the great Dyke to the north, 

 until it is entirely lost. This is not however to be taken generally, but 

 as being the case in this particular district, for in many places the 

 Sandstone, or Post, is the cover of the Coal for a very considerable dis- 

 tance from the Dyke. 



It might be supposed that where the Sandstone thins, and eventually 

 runs out, the superincumbent bed will then become the cover or roof 

 of the Coal, but it is necessary to remark that this is not the case. At 

 a considerable distance before the Post runs out, a thin bed of Argillo- 

 bituminous Shale interposes itself between the Coal and the Sandstone ; 

 at first extremely irregular in its occurrence, but gradually becoming 

 more continuous towards the north, until it ultimately becomes the ex- 

 clusive roof of the Coal. While this bed of Shale is from 1 to 2 feet in 

 thickness, it is of a uniform Argillo-bituminous nature, but when its 

 thickness increases, and the Post is split into thin layers, about 12 or 

 13 inches of the lower part of this bed still retain the same character 

 of " Blue Stone" (No. 1), but the upper part becomes distinctly sepa- 

 rated by what is called by the miners " the black parting," and be- 

 comes of a more silicious nature, with innumerable minute specks of 

 Mica, or what is called " Blue Metal Stone" (No. 2), which seems to 

 pass insensibly into, first micaceous, and then compact " Post," a Sand- 

 stone, being probably a continuation of the Main Post. It was in these 

 strata the immediate cover of the Coal, and at a depth of about 48 fath- 

 oms from the surface, that the fossil stems were found ; and the follow- 

 ing is a section from the Coal upwards of the beds inclosing these 

 reinains, specimens of which, numbered as per margin are presented. 



No. Ft. In. 



Coal High Main Seam 6 



1. Blue Metal, or Argillo-bituminous Shale 2 3 



2. Blue Metal Stone, 1 1 



3. Parting Silicious 



Carried forward 9 4 



