200 Mr. Maclauchlan's Notes to accompany a Geological Map 



At a place called " the Dunder Hole" (Thunder?) on Bishop's-wood, a stream sinks through the 

 strata with a considerable noise, and emerges again in the valley to the south, a little above Bishop's- 

 wood House, where it is perhaps thrown out by the bed which underlies the great stratum of lime- 

 stone at Ruerdean. A similar occurrence takes place in the limestone about a mile to the south 

 of English Bicknor. 



Near the "Dunder Hole" is a bed of gravel in the position of the millstone 

 grit, but it is very different in appearance from the grit in the Forest. It has 

 much the character of diluvium ; and if there were an aperture through which 

 the Wye at any elevation could have deposited it, the similarity in appearance 

 would lead to such a conclusion. 



There is a small outlier of the carboniferous limestone at Penallt Common, 

 about three miles south of Monmouth ; it dips conformably with the sandstone 

 on which it rests, and is so valuable for manure in that clay and sandy coun- 

 try, that it is nearly worked out, though it must have covered two acres in the 

 first instance. There is no appearance of the coal strata. 



Coal Measures. 



In tracing the outline of the coal measures, Mr. Mushet's Section, pub- 

 lished in the 1st volume of the Geological Transactions, 2nd Series (PI. .32, 

 No. 5.), has been particularly useful, as it crosses the field in alm0b>t the only 

 direction, which includes all the strata. 



The beds of coal may be conveniently divided into two series ; 1st, the rock 

 beds, or those separated by a coarse sandstone, including all the seams from 

 the lowest or " Trencher," to the " Brazilly," and, 2ndly, the argillaceous 

 heels, or those above the Brazilly, and characterized by shale. 



Of the lower series, the outcrops of only the " Trencher" and the "Cole- 

 ford High Delf " have been traced almost continuously around the field ; but 

 these beds differ in their relative distances and qualities ; and it is said that 

 one increases in value where the other decreases. 



At Lydney Park the "Coleford High Delf" and "Trencher" are not well 

 defined at the angle, the outcrops being there very obscure. Prom this point 

 they increase in dip, with the rising ground, to Bream's Eaves, and afterwards 

 decrease again, but at Gorse Hill and Coleford Meend, they become hori- 

 zontal, and crop out to the westward, more from the declivity in the ground 

 than from their ascending position in that direction. This observation will 

 apply to their outcrops on the north also, from a little south of Symmonds' Gate 

 to Joyford and Carter's-piece Meend. 



At this place the line of division*, ranging from Bic Slade to Carter's-piece 



* This line is nearly parallel to what is assumed to be the line of greatest depression, or central 

 line of the field or floor, which rises to the north at about 10°. 



