112 Mr. F. Forster's Observations on the South Welsh Coal Basin. 



intersects the strata near Pontneath Vaughan, and produces the same 

 effect, increasing the width of the Basin, on its eastern side. At about 

 half a mile to the west of Penllwyngwyn (see the section, division No. 

 2), a similar Dyke of 90 or 100 fathoms downthrow to the eastward, re- 

 peats and extends the Penprys and Clyngwernon seams in that direction, 

 the former of which would not otherwise have reached the point where 

 the section crosses the Basin ; and, following the water-level course of 

 the strata to the eastward, from the termination of division No. 2, a 

 number of Dykes, of less magnitude, occur within the space of three 

 miles. There are many others, of greater or less consequence, but my 

 observations in this extensive disti'ict, have not been so minute as to 

 enable me to give their direction and effects. It may be observed, that 

 the great inclination of the strata, and the numerous out-breaks of the 

 seams of Coal afford the means of ascertaining the downthrow of the 

 north and south Dykes, which may be calculated from the extent to 

 which the strata are protruded or extended forward on their downcast 

 side. In addition to these Slip Dykes, the Welsh Coal Seams are liable 

 to irregularities, which, though less decisive in their effects, are of a 

 much more troublesome nature ; they are called by the miners " Rolls," 

 and their occurrence is marked by the irregular and variable thickness 

 of the seam of Coal affected by them, which, in some cases for se- 

 veral hundred yards together, will be repeatedly contracted and enlarged 

 from 4> or 5, and even from 10 or 15 feet, to 2 or 3 inches ; the quality 

 and texture of the Coal being almost entirely destroyed. These Rolls, 

 unlike the Slip Dykes, only affect, generally speaking, particular seams, 

 and do not extend equally to the overlying and underlying strata. Al- 

 though Trap, irregularly stratified, occurs extensively among the lower 

 members of the Coal formation in Pembrokeshire, I am not aware that 

 any Trap Dykes traverse the eastern or principal part of the Basin.* 

 Fire Damp. The quantity of Carburetted Hydrogen Gas, occurring in 



* The Pembrokeshire district is, I believe, intersected by one or more Trap Dykes. An 

 account of the effect produced on the Stone Coal in contact with the Dyke, could not fail 

 to be interesting. Judging from the nature of the Coal, it is very probable that it will be 

 little, if at all, changed in its appearance. 



