Sept. 4, 1879] 



NA TURE 



449 



crust which terminated the physical geography of the Devonian 

 age, three elevated tracts of land crossed the kingdom from west to 

 east, and that there were mountainous regions running northwards 

 and north-westwards, including North Wales, Western Ireland, 

 and much of the North Atlantic. 



The southern high land barrier passed somewhere in the 

 direction of the Bristol Channel, and then to the east and slightly 

 to the south, having a somewhat definite continuation with 

 the Ardennes. The central barrier, or high land, passed from 

 Shropshire eastwards by Leicester, and then to the coast ; and 

 the northern was formed by hills in the present lake district, 

 extending eastwards. On the south of the southern high land, 

 the marine Devonian accumulated in a coral sea, and to the north 

 of it and between it and the central barrier the old red lakes 

 obtained their water supply and sediment from the Welsh hills 

 of the period. North of the central barrier, interrupted lakes 

 and land occurred, and also to the north of the northern barrier. 

 The dry land and the barriers and hills were formed by sub-rocks 

 of Silurian and Cambrian age. 



There is no evidence to indicate that the southern barrier was 

 of great height at the end of the Devonian period, but there is 

 some which points out that the first physical change which 

 initiated a new aspect of nature — the carboniferous — was a 

 general subsidence of the region. The coral reefs sank below 

 the bathymetrical zone of the composite forms, and the sea 

 breached the barrier. 'I"he southern old red lake began to 

 have its waters impregnated with salt, and its great ganoid fish 

 were replaced by the cestraciont sharks of the age. These left 

 their remains in the bone bed at the base of the lower limestone 

 shales, which are the earliest of the carboniferous series there. 

 The irruption of the sea appears to have taken place to the north 

 of the central barrier also, and the subsidence was great there, a 

 limestone with some sandy strata forming gradually. In the 

 north and north-east, in the present district of the Tweed, 

 deposits collected in shallow water, and vegetation grew which 

 formed the coals at the base of the great Scour limestone. 



On the same and on slightly higher horizons are the coals of 

 Fallowfield, Tindal Fell, and Heskett. These are the earliest 

 evidences of the carboniferous vegetation, and it was doubtless 

 in full vigour whilst marine conditions existed to the south. 



Probably the high lands constituting the barriers were not 

 covered during the subsidence, which permitted the accumulation 

 of the marine deposits of the carboniferous limestone age. For 

 close to the coal-fields near the central barrier, and which rest on 

 upper Silurian rock, borings here found the remains of carboni- 

 ferous plants on the palaeozoic rock without the intervention of 

 any sediments. 



Now the depth of the deposit of limestone about this central 

 barrier is great, and the question arises how was it produced in 

 the immediate proximity of land which was not covered by sea, 

 and which does not appear to have sunk contemporaneously with 

 the sea floor close by ? Sinking along definite lines bounded by 

 faults is the only means by which this can be explained ; and 

 this suggestion, which was a favourite topic with Pliillips, is all 

 the more probable, when it is remembered that the area of 

 accumulation to the north of the barrier was one of vast subsidence 

 during the consecutive ages of the grits and coal measures, whilst 

 there was land still further north. If the stability of one and 

 the instability of the other are not conceded, the original height 

 of the barriers must have been stupendous and beyond example, 

 so far as the size of their bases is concerned. 



There are many examples of v^hat I resolved to call in a 

 presidential address before the Geological Society areas of com- 

 parative instability and which relate apparently to radial upheaval 

 subsidence along long lines of country where movement has been 

 rare. An instance on the grandest scale is seen in the history of 

 the Himalayas in relation to the peninsula to their south and 

 scrath-west. For whilst this last area was land during a vast 

 age, that of the Himalayas was repeatedly a marine tract, and 

 suffered subsidences and elevations. 



Still further north and beyond the northern barrier, in the 

 Scottish area, carboniferous plants lived a little later, and after 

 a subsidence which permitted the lower calciferous series to 

 accumulate. The lowest coals of the basin of the Clyde are of 

 this age, and the accompanying clay, ironstone, and the fresh 

 water limestones and gigantic fish of liurdie House are all 

 indications of terrestrial conditions. All these evidences of 

 carboniferous vegetation occur in the geological horizon of the 

 carboniferous limestone and Yoredale scries. 



Never entirely free from sandy impurities the carboniferous 



limestones north of the central barrier gradually became covered 

 witli a thick ai-enaceous series containing here and there marine 

 fossils and traces of coal plants. These are the Yoredale strata, 

 which consist mainly of the sediments of a somewhat distant 

 north-westerly land, the plants of which were carried to sea by 

 rivers and deposited here and there on the sea floor. It would 

 appear from the evidence collected by the Geological Survey 

 that, after a very considerable thickness of these rocks had 

 collected, either a filling up of the shallow sea or a slight 

 upheaval of the floor occurred, for denudation of their surface 

 happened, considerable depressions and ridges being produced on 

 it. On those spaces and ridges, and indeed on the whole surface 

 of the Yoredale rocks, collected strata which are popularly 

 called the millstone grits, so well seen west of Sheffield. All 

 the depths of this great land wreckage, consisting of silicious and 

 felspathic sandstones and shales, accumulated on a sinking area, 

 some near land and the rest in deeper places. And here and 

 there coal seams are found intercalated, being evidences of the 

 existence of contemporaneous vegetation. Some of them are 

 workable, and others are only valuable as evidences of the exist- 

 ence of the vegetation of the age ; many are placed on a hard 

 silicious or ganister bed, but some have an underlying fire-clay. 

 They are very usually covered with deposits containing goniatites 

 and aviculopecten, which doubtlessare the remains of marine 

 organisms. 



Admitting, therefore, that some of this millstone grit coal may 

 be the result of the drifting and sinking of the vegetation from 

 off lands rather remotely situated, it is still evident, from the 

 existence of the under-clays elsewhere, that some of the grits, 

 by silting up, or by slight upheaval, above sea level, formed 

 the subsoil of swampy ground on which vegetation grew. This 

 approach of the millstone grit sea floor to above sea level was 

 decided enough in the region of the great coal-field around us, 

 for a conglomeratic rock — the rough rock — occupies a somewhat 

 definite horizon on the top of the series. 



This rough rock collected in shallow water, and it is impor- 

 tant to the geological surveyor, for it formed the base on which 

 the coal measures, proper, rest ; and it is suggestive to the physical 

 geologist that a general and wide, but not great, upheaval took 

 place which remod'ed the ocean of the day further off, and 

 which determined a total change in the direction of sediment- 

 depositing currents. 



Hitherto the greatest thickness of the sediments of the millstone 

 grit age had been towards the north-west, and the direction of 

 the currents had been from north-west to south-east, but 

 subsequently, as has been suggested from very strong evidence 

 by Sorby, the depositing currents of the next age had no very 

 definite direction. But the carboniferous land of this part of 

 Europe was not yet remote from the sea. Much of it was on 

 the borders of estuaries, and the aspect of nature was probably 

 that of wide flats of grit covered usually by terrestrial vegetation 

 and occasionally overwhelmed by sea. In fact, both practically 

 and theoretically there is much difficulty in separating the mill- 

 grits from the lower coal measures. The lower measures contain 

 some thick and widely-spread sandstones, and the important 

 coal seams, in some instances, rest on a hard ganister bed, and 

 in others on a fire-clay. And to add to the similarity of the 

 deposits of the upper grits and lower coal measures, marine 

 fossils, such as species of goniatites, aviculopecten, and posido- 

 nomya, are intercalated above the coals. But the evidences of 

 marine invasion ceased as the deposits accumulated, and more 

 perfect terrestrial conditions arose. The EUand flag-stones, for 

 instance, such prominent features to the west of this town and 

 in the neighbourhood of Halifax, are fresh-water deposi's, and 

 are undoubtedly accumulated in an under-clay indicative of 

 terrestrial conditions. 



In the region north of the northern barrier successive coal 

 seams and impure limestones and fire-clays occurred during the 

 age of the depositions of the English grits, and then a thick 

 fossiliferous sandstone was followed by the upper coals of 

 Mid-Lothian. 



All the minor upheavals and upsiltings of this long age were 

 subordinate to a progressive general subsidence, in which the 

 central and northern barriers were slightly implicated, and this 

 extraordinary crust movement was to continue during the 

 accumulation of over 3,000 feet of coal measures and other 

 dei>osits, all subacrial in their method of formation, or having 

 collected in shallow water or swampy ground. These products 

 of denudation and of organism succeeded each other time after 

 time ; great gravels, shales, and sands were intercalated, and 



