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forests, with their "underclays " or ancient soils on which they flourished ; while 

 beds of impure Coal or bituminous shale bespeak the preponderance of muddy silt 

 among the drifted vegetation that slowly decayed and dropped to the bottom of 

 the estuary of deposit. I would refer those who wish for further information on 

 this very interesting subject to Paijes Advanced Text-Bonk of Geoloyy, 5th edition,* 

 to which valuable work I am indebted for the theories on the formation of Coal 

 now brought to your notice. 



Very much that is interesting might be said of the Basaltic formation which 

 crowns these hills, but time will not now admit of it. You will have seen by the 

 section figured in Murchison's Siluria, a vertical mass of Basalt rises up through 

 the strata, and overspreads the hill tops. This is a more compact form of Doler- 

 ite, and has been molten under intense heat, and could again be reduced to that 

 condition ; it consists essentially of Augite and Felspar, the former predominat- 

 ing, with a considerable admixture of iron. It is close-grained, hard, usually 

 black, and frequently columnar, which structure is the result of cooling. It 

 would probably be of Permian age, intruded into and overflowing the carbon- 

 iferous rooks in Permian times. The fact of Coal being now found on these hills 

 is chiefly attributable to the hard Basalt having, by its resistance to denudation 

 and other eroding influences, seized the Coal and held it in its present position. 

 How much of it was washed and worn away before this condition arose there are 

 no means of ascertaining, but it is known that the Coal Measures here are of 

 trifling thickness and importance as compared with those of South Wales ; and 

 yet the supjjosition is perfectly reasonable that at some period they were united. 

 Coal Measures vary greatly in thickness, and most geologists consider that it is 

 seldom, or perhaps, never, that we obtain the full thickness, because great denud- 

 ation has in nearly all cases affected the strata. In South Wales the total thick- 

 ness of the series has been reckoned at from 10,000 to 12,000 feet. It has been 

 estimated that the sediment increased two feet in a century, and that it would 

 probably take 1,000 years to form a bed of Coal one yard in thickness, and a cal- 

 culation has been made that the deposits forming the South Wales coalfield might 

 have been accumulated in 640,000 years. 



*This work, revised and in great part re-written by Chas. Lapworth, has now, iS 

 reached its twelfth edition. 



