1848.] 



- THE CIVIL ENGINEER AND ARCHITECT'S JOURNAL. 



313 



water of the artesian well, Trafalgar-square, London, contains in each 

 gallon about 20 grains: that at Combe and Delafield's Brewery, 12 7 ; 

 that at Wolverhampton Railway Station ; one lately sunk at Southamp- 

 ton, for supplying a private manufactory, 40. May it not be asked, 

 whether the subject of the suitableness of waters in general for the 

 various purposes to which they are spplied— be it in mannfaclories or for 

 steam-engines, domestic purposes or drinking — is not worthy of a greater 

 share of scientitic attention than it has hitherto commandtd ! 



Geology of Socth Wales. 



On (he Geology of Portions of South Wales, Gloucestershire, and Somer- 

 setshire. By Sir n. T. De la Beche. 



The rocks of this district have originated in several distinct ways: some 

 have had a mechanical origin, and consist of the detritus of older rocks 

 broken into fragments or reduced to powder, and brought down from the 

 land by rivers, or worn by the breakers from the coast ; others have been 

 deposited from a state of chemical solution, like some of the limestones ; 

 and some have been formed almost entirely from the aggregation of organic 

 remains. The materials obtained from the destruction of the older rocks 

 have been employed over and over again in the formation of those of later 

 date. 



1. The most ancient strata of the district have been denominated Cam- 

 brian and Silurian : the former may be seen in Pembrokeshire, towards St. 

 David's. They contain the earliest fossil remains which have been dis- 

 covered. Numerous volcanoes appear to have been active at this period, 

 ejecting quantities of ashes which, falling into the sea, entombed the ani- 

 mals living on its bed. Even at this early period there may be discovered 

 indications of portions of the sea's bed having been upheaved so as to 

 form dry land or islands. Subsequently a depression took place, and an 

 accumulation of sand was spread over the whole, constituting what is called 

 the Caradoc sandstone. After many thousands of feet of materials had 

 thus been accumulated and become consolidated, another contortion and 

 folding of the strata took place, followed by a change in the nature of the 

 materials deposited in the sea. 



2. The second series of deposits constitute the Old Red Sandstone, which 

 consists mostly of detrital matter, but contains occasional beds of impure 

 limestone (cornstone), and in its lower part there is a great thickness of 

 marl, also coloured red by per-oxide of iron, and occasionally streaked with 

 blue and green where the iron has been reduced to a lower state of oxida- 

 tion by the presence of decomposing vegetable matter. The upper beds 

 consist of coarse sand and gravel cemented together and forming a hard 

 conglomerate ; the multitude of quartz pebbles, derived from veins, indicates 

 an immense destruction of older rocks. No fussils are found in this forma- 

 tion, because sands are always barren, and per-oxirie of iron is fatal to ani- 

 mal life when it exists in excess ; but in the cornstones a few remarkable 

 fishes {Cephalaspis, life.) have been obtained. These rocks appear to have 

 been formed near a coast, whilst at a small distance in Cornwall and Devon 

 the sea was depositing fine sediment, was free from the injurious per-oxide, 

 and abounded in organic life. The Old Red Sandstone is sometimes con- 

 formable to the underlying Silurian rocks, — at others unconformable ; and 

 in the Valley of the Towy it begins to overlap the Upper Silurian and rest 

 upon the Lower Silurian rocks ; further west, it is itself overlapped by the 

 coal-measures which there rest on the Silurians. 



3. After this another great change took place, and the sea deposited car- 

 bonate of lime, forming the carboniferous limestone, well shown on the 

 coast of Pembrokeshire. The lower part of this serirs consists of sand- 

 stones and shale, in which the remains of fishes occur in abundance. Mol- 

 lusca also appear; and soon the abundance of organic remains becomes so 

 great that whole strata are formed of their remains: indeed, the carbonate 

 of lime seems to have been chiefly produced by the agency of animal life. 

 This limestone, which is sometimes 2,000 feet thick, dwindles to 70 or 80 

 feet towards Haverfordwest, and does not appear to have extended far to 

 the north. Here a change of mineral character takes place in the coal- 

 measures, originally consisting of mud, sand, gravel, and accumulations of 

 vegetable matter. The lowest division, millstone grit or farewell rock, is 

 usually a white quartzose sand, but sometimes a calcareous mudstone with 

 organic remains, the equivalent of the culm of Devon. Above this were 

 formed beds of mud and sand, with occasional beds of vegetable matter and 

 carbonate of iron; these beds diminish in thickness from Merthyr Tydvil to 

 Pont-y-pool ; and are wanting in Dean Forest, hut exist in the ISristol field. 

 With respect to the origin of the coal in this district, there is evidence that 

 it originated in accumulations of vegetable matter which grew on the spot. 

 The conditions under which the beds of coal occur have been described 

 minutely by Mr. Logan ; under each coal seam is a bed of sandy clay, full 

 of the fossil plants known as Stigmariis, and which Mr. Binney has shown 

 to be the roots of another plant, the Sigillaria, equally abundant in the coal, 

 which must have grown in swamps near the sea. After each great accumu- 

 lation of vegetable matter, the land seems to have subsided, and the sea 

 flowed ill, bringing sand and mud and marine shells; again marshes were 

 formed and fresh accumulation of peat and plants, to he in turn covered by 

 silt from the sea. Evidence of the local origin of the coal is also afforded 

 by the frequent occurrence of fossil trees with their trunks erect and their 

 roots spreading out in the clay below; several of these trees, each 14 or 15 

 feet high, were discovered at the head of the Taw Valley ; the outside of 



their trunks appears to have been originally hard and to have resisted the 

 action of water for some time, but their interior was soft and soon became 

 hollow and filled with mud, which is regularly stratified ; the sandstone on 

 the outside of the trees also bears traces of the rippling of the water around 

 them. The iron ore of the district occurs in the form of nodules of argil- 

 laceous ironstone, lying in courses ; the cracks in these nodules being filled 

 with carbonate of iron just as those in the cement-stones (septaria) of the 

 lias are filled with carbonate of lime. One of the phenomena of the coal dis- 

 trict is the occurrence of cracks, attended with the displacement of the 

 beds on either side ; these faults are numerous, and amount in one instance 

 to 2,400 feet ; the cracks are sometimes wide, whilst at others the sides are 

 in close contact. Many of the faults appear to have been formed before the 

 deposit of the raagnesian conglomerate ; but others appear to have been 

 formed at almost every subsequent period. In some instances beds of coal 

 seem to have been partially washed away before the accumulation of the 

 succeeding bed, giving rise to spurious faults, such as that called the 

 " Horse" in the Forest of Dean. 



4. At the conclusion of the coal period, all the existing rocks appear to 

 have been squeezed and contorted not only in Britain, but over a great part 

 of Europe, a new deposit of detrital matter began to be formed, similar to 

 those before the coal period, and called by way of distinction the New Red 

 Sandstone. Where this formation approaches the older rocks it puts on the 

 appearance of a shingle bed, in which the detritus of the older rocks is ce- 

 mented together by carbonate of lime and magnesia, hence termed the mag- 

 nesian conglomer.ate. These fossil beaches are thickest on the south-west 

 and west flanks of the Mendip and other hills, indicating an open ocean 

 and prevalent winds in that directron. In the red sandstone and marls 

 formed at the same time, but further from the coast, there are no traces of 

 animal life ; but as the red stain disappears from the rocks, towards the 

 conclusion of the period, remains of fishes and shells appear. 



5. Further suljsidences took place ; the sea, now freed from the per- 

 oxide of iron, swarmed with animals of extraordinary form and structure. 

 We still trace its boundaries in Glamorganshire and the Mendips by beds of 

 rolled pehldes from the subjacent rocks, and close to these sheltering coasts 

 the remains of marine saurians abound in the consolidated mud and lime- 

 stone {Lias), along with the bones of the flying Pterodactyle. Somewhat 

 later, great beds of oolitic limestone were accumulated in the sea, which 

 now constitute the Cotteswold Hills and their extension to Bath and Dorset- 

 shire. 



6. No further history is afl'orded by this district until comparatively rao- 

 ern times, when we find evidence of subsidence beneath the sea and of 



agencies by which the present form of the surface was accomplished. The 

 presentland must have been at least 1,500 feet lower; and, therefore, nearly 

 all under the sea. There is also evidence that the climate became cold, that 

 there were glaciers in the mountains of North Wales and icebergs floating 

 round the shores, carrying blocks of stone and gravel and presenting all the 

 phenomena of polar regions. The sea also accumulated beds of clay, in 

 which the few existing shells are of Arctic character. Still later, the land 

 must have risen again above the sea to an elevation greater than it now has, 

 for we find submarine forests fringing all the shores of Europe from Spain 

 to Norway. Of this, one of the best examples occurs in Swansea Bay, 

 where the stumps of oak and alder may be seen at low water, 20 or 30 feet 

 lower than they could have grown. 



" On the Relative Position of the various Qualities of Coal in the South 

 Wales Coal-field." By S. Benson, Esq. 



The coal is of three kinds : 1. bituminous, the small of which will coke ; 

 2. free-burning, the small of which will not coke, but which burns 

 with great rapidity and a considerable volume of flame ; 3. anthracite or 

 stone coal. These three pass into one another imperceptibly ; the same 

 vein of coal changing gradually from bituminous to free-burning, and from 

 this to anthracite. 



1. The coal beds which crop out on the south side of the basin are highly 

 bituminous, becoming less so towards the north. The five-foot vein, exten- 

 sively worked near Swansea on the south rise, is highly bituminous, — on the 

 north rise, within a distance of two miles, it becomes free-burning. The 

 various beds also differ considerably in their bituminous qualities and com- 

 mercial value. 



2. The free-burning coals occupy a tract running north-east and south- 

 west through the centre of the coal-field. Those which are intermediate 

 between the free-burning and bituminous are perhaps better adapted than 

 any for smelting purposes, — and in the neighbourhood of Merthyr form the 

 chief supply for the blast furnace, being either used raw or the large only 

 being coked in the open air. The pure free-burning coals are less adapted 

 for smelting, but are preferred for steamers from their readiness of combus- 

 tion and the absence of clinkers in the grate. Free-liurning coals are 

 admitted to government contracts from the following places : Llangennech, 

 Camerons, Graigola, Briudowey, Resolven, and Aberriare. 



3. The northern side of the basin is occupied by the anthracite, which 

 graduates through the various " culms" into the free-burning coal. In 

 Pembrokeshire the coal is all anthracite. Taking the area of the Glamorgan- 

 shire coal-field at 750 square miles, it is estimated that xjths of this area is 

 occupied by bituminous and free-burning coals, and the remainder by culm 

 and anthracite. It appears that the beds of coal on the south crop lose 

 their bituminous qualities gradually as they dip to the north ; so that if on 

 a section lines are drawu to show the boundaries of the qualities of coal 



41 



