490 



NATURE 



\ April 1 8, 1872 



the marine fauna of the Silurian and other epochs ; and while now 

 they occur only in the very deepest water, they were formerly 

 found crowded in the shallower seas. The inquir;-, 1' ;f.:re, 

 suggested itself to the Professor as to the reason of thii dJiereuce, 

 and he makes the suggestion that in the progress of the earth's 

 growth we may look to such displacement of conditions favour- 

 able to maintaining certain low types as may recall most fully 

 the adaptation to former ages, and that the deeper waters of the 

 present constitution of our globe possibly approximate the con- 

 ditions of animal life in the shallow seas of former ages as nearly 

 as anything can in the present order of things on the earth. Tlie 

 depth of the ocean alone, he thinks, can place animals under 

 the high pressure which the heavier atmosphere of the earlier 

 period afforded. But as such pressure cannot be a favourable 

 condition for the development of life, it is to be expected that 

 the lower forms only will occur in the deep seas. Other causes 

 acting in the same direction are the decrease of light in the 

 greater depths, the smaller amount of free oxygen, the reduced 

 amount and smaller variety of nutritive substances, &c. He does 

 not think, however, that facts warrant the conclusion that any of 

 the animals now living are lineal descendants of those of the 

 earlier ages, nor that we may justly assume that the cretaceons 

 formation is still extant, notwithstanding the similarity of forms. 

 It would be just as true to nature to say that the tertiary period 

 is exhibited in the tropics, on account of the similarity of the 

 Miocene mammalia and those of the torrid zone. — The ninth 

 number of the illustrated work on the butterflies of North America, 

 in course of publication by Mr. William H. Edwards, has just 

 made its appearance, and we are informed that the tenth num- 

 ber, to appear very shortly, will conclude the first volume. This 

 number, like its predecessors, is accompanied by a great many 

 quarto plates in the highest style of pictorial excellence, depict- 

 ing some extremely beautiful species and varieties of butterflies. 

 Among these are three varieties of Papilio A /ax, namely, tVah/iii, 

 tdamoniJcs, and MarceUus. Mr. Edwards, in his paper, 

 makes some judicious remarks upon the uncertainty that exists 

 in regard to the true character of many butterflies which some 

 naturalists consider as perfectly distinct species, and others as 

 mere varieties. He takes the ground that the only way of coming 

 to a satisfactory conclusion is to breed them, and ascertain 

 whether the eggs from the same female develop similar lai-vce or 

 not, and whether these, even if different, produce the same per- 

 fect insects or diliferent ones. The attempt at discriminating 

 from the perfect insect alone he considers extremely unsatisfac- 

 tory. 



ANNUAL ADDRESS TO THE GEOLOGICAL 



SOCIETY OF LONDON, FEB. 16, 1872 



By J. Prestwich, F.R.S., President 



(Concluded from p. 472.) 



TT has been urged as a fatal objection to the discovery of coal 

 in the sou'h-east of England, that the Coal Measures become 

 unproductive and thin out under the Chalk, as they range from 

 Valenciennes towards Calais, and, therefore, that the coal- 

 trough or basins end there. It is perfectly true that the Coal 

 Measures do thin out between Bethune and Calais, but not in the 

 sense of their dying out owing to their deposition near the edge of 

 a basin. In that case, each seam, each stratum, would gradually 

 become thinner and disappear ; but such is not the fact. None 

 of the beds of the Belgian coal-field are thick. The average 

 does not exceed 2\ feet. At Valenciennes it is the same ; 

 whereas M. Burat stales the mean thickness of the beds actually 

 increases westward of Bethune to more than 2I feet. With re- 

 spect also to the extreme end of this basin, the lower beds there 

 brought up correspond with the bottom beds of the Hainault 

 basin, where the lower 650 feet consist of unproductive measures. 

 The thinning-out is, in fact, due to denudation, just as the 

 Bristol coal-field thins out at Cromhall to resume in the Forest of 

 Dean, or the coal-field of Liege thins out at Nameche to resume 

 at Namur in the great field of Charleroi and Mons. 



The deterioration of the coal in the small coal-field of Hardin- 

 ghen, near Boulogne, has also been adduced against the occurrence 

 of worksbl; coal in South-Eastern England, but Mr. Godwin- 

 Austen has shown that this Hardinghen coal-field is one of those 

 small local developments of coal-bearing strata intercalated in 

 the Mountain Limestone, and is of older date than the great 



Belgian coal-field. It has, therefore, no bearing on this part of 

 the question. 



Another objection to which much weight has been attached is 

 that the coal-field of Bath and Bristol forms an independent 

 basin, cut off both on the east and on the west by ridges of i' 

 Millstone Grit and Mountain Limestone, so that there is an end ■ 

 of the eastern extension of the Coal Measures. This is quite 

 correct as far as regards the western edge, and is probably the' 

 case on the eastern, although as the edge of the basin is there 1 

 covered by Secondary rocks, some uncertainty still exists about? 

 the disposition of the Palteozoic rocks under them. Admitting, 

 however, the basin to be complete and isolated, that is no proof 

 that the older Palxozoic rocks prevail exclusively to the east ;i 

 for the Coal Measures of the Somerset basin maintain their full 

 development to the edge of the basin, and are there cut off by 

 denudation, and are not brought to an end by thinning out. 

 They form really part of a more extended mass, of which we 

 have there one fragment ; while on the west another portion 

 exists in the Welsh basin, and another in the newly discovered 

 small basin of the Severn valley. .1 



This last basin is entirely covered by the New Red Sandstone ;i 

 and as the Welsh basin is bounded on the east and the Bristolt 

 basin on the west by Mountain Limestone, the same argument ij 

 as the one above might have been used to show the impossibility! 

 of coal occurring in this intermediate area. --.} 



But the fact is, it is the very nature of this great line of dis- ; 

 turbance to have minor rolls and flexures of the strata at, or.j, 

 nearly at, right angles to it, and so causing breaks in the coal-i; 

 trough, which would otherwise flank it without interruption ;tjj 

 thus the Aix-la-Chapelle coal-field is separated by older rocksfj 

 from that of Liege, which is again separated by a ridge of Moun-rf 

 tain Limestone from that of Ilainaut. So in the case of south-, ji 

 western England, we have the several basins of South Wales, J 

 Severn Valley, and Bristol, separated by tracts of Mountainij 

 Limestone and Old Red Sandstone, the extremes of the inter- 1 

 vening belts of older rocks being two miles at Nameche and i 

 eighteen miles in Wales. These barriers are clearly only local, .; 

 and the division of the Coal Measures into separate basins ap- 

 pears to be their ordinary condition along this great line of '" 

 disturbance. The length of the two known portions of the axis \ 

 included between Pembrokeshire and Frome, and between || 

 Calais and Westphalia, is 472 miles, and in this distance we find ij 

 eight separate and distinct coal-fields. The combined length of 

 these eight coal-fields is about 350 miles, leaving about 122 ' 

 miles occupied by intervening tracts of older rocks ; so that '■ 

 nearly three-quarters of the whole length is occupied by coal- • 

 strata. I consider that a structure which is constant (so far as 

 the axis of disturbance can be traced above ground) is, in all 

 probability, continued under ground in connection with the range 

 of the same line of disturbance ; and I see no reason why the j 

 coal-strata should not occupy as great a proportionate length and 1 

 breadth in the under-ground and unknown, as in the above- f 

 ground and explored area. 1 



With respect to the possibility of denudation having removed 

 the intervening Coal Measures, enormous as the extent of de- 

 nudation must have been previous to the deposition of the Per- 

 mian strata, we cannot admit its exceptional action in this case. 

 Denudation has removed from the crest of the Mendips a mass 

 of strata possibly equal to two miles or more in height, and 

 from that of the Ardennes as much as three or four miles, and j; 

 it has also worn extensive channels between many of our coal- ! 

 fields, so that the power of sucli an agent cannot be denied. 

 But it is a power of planing down exposed surfaces rather than of ■; 

 excavating very deep troughs. Notwithstanding its immense : 

 planing-down action on the Mendips and Ardennes, deep troughs j 

 of Coal Measures are left flanking their northern slopes. These 

 troughs descend to more than a mile beneath the level of the 

 sea ; and I do not think it probable that those underground in- 

 termediate portions of the trough where the axis is lower, would 

 have suffered more than those on the higher levels, unless it 

 were to the extent caused by the later denudation which pre- 

 ceded the Cretaceous period. But this would not affect the main 

 bulk of the Coal Measures. The Belgian coal-field, which was 

 exposed to the action of both these denudations, still retains 

 vast proportions. 



I may remark that the pre-Cretaceous denudation was very 

 irregular in its action. At one place near Mons the Chalk and 

 Tertiary strata are above 900 feet thick ; whilst at another, on 

 about the same level, and at but a short distance, they are not 

 100 feet thick — an old under-ground ^ hill of highly inclined 



