62 



THE BRISTOL I OALFlELD. 



difference, there is no carlonaceous matter between the thin bedB of 

 the latter. If. however, we examine the chemical combination of 

 these two, we find a marked difference. In No. I the silica is 10-37 

 per oent. Less than in No. 5. The alumina is 4-78 in excess ; and 

 manganese and sulphur occur in No. 1. (Compare with Doxall.) 



Now. if we compare beds Nos. 2 and 1. we find the physical condi- 

 tions again similar, with the exception that there is visible organic matter 

 mixed with No. 1, and we find the same thing applies to the silica and 

 alumina as in the previous comparison. We may also compare bed 

 No. 3 with No. 1 with the same result, but to a lesser degree. We 

 Bhould not expect to get bo decided a difference when comparing No. 

 4.* as the carbouaceous matter is much less, and is not associated in 

 the same way as in No. 1. In No. 4 it has the appearance of having been 

 deposited with the sediment which formed the rock in No. 1 ; it is in 

 Htnttiiied layers, the rock having been deposited over in alternate beds. 



Now. if we take the analysis of those semi-argillaceous beds which 

 occur over seams of coal and in coal-bearing strata called duns, wo 

 find that the same constituents are present as in the arenaceous rocks, 

 i.e., the grits, the chief difference being that there is more alumina in 

 the former. This the following analysis will show : — 



Analysis of Shale anu Duns. 



Now. we have found that the grits when in contact with carbonaceous 

 matter become more argillaceous, that is to say, the proportion of 

 alumina is greater ; and if this process were to continue, rocks would 

 be formed of the composition of shale and duns. Let us now see how 

 this chauge has been brought about. My analyses have shown thai 

 the grits contain silicates, and there is no doubt that when first depo- 

 sited they were in greater proportion; we have then to consider what 

 would arise if this rock-forming sediment was deposited with, or over, 

 masses of vegetation. 



The vegetation would decay, and carbonic acid gas would be 

 generated as one of the results. Now the offect of this gas on 



• In the analysis of bed No. 4, the organic matter was scraped off the stone, so 

 that the i>crcentace of oarbou giTen in the analysis is no guide as to the actual 

 MDOUai iu the bed. 



