196 



SCIENTIFIC NEWS. 



[Aug. 31, 1888. 



comes the Secondary, or Mesozoic system, from mesos, 

 middle, and soe, life ; and finally the Tertiary, or Caino- 

 zoic, from kainos, recent, and zoe, life. In considering 

 the geology of Bath it will not be necessary to refer to 

 strata later than the Mesozoic. 



At the base of the Palaeozoic series we get rocks to 

 which the name Laurentian was given by Sir William 

 Logan in 1854, by which term he intended to distinguish 

 a thickness of something like 30,000 feet of strata, typi- 

 cally developed in the Laurentine Hills of Canada. Rocks 

 which are probably the representatives in time of the 

 Laurentians of Canada occur in this country, and are 

 generally spoken of as the Archaean, a term proposed 

 by Professor J. D. Dana, in 1874, to include the Lauren- 

 tian group and rocks contemporaneous with them. The 

 nearest locality to Bath at which rocks of certain Archaean 

 age are exposed is in the Malvern Hills. 



Next in the order of succession come the Cambrian 

 series, in which life is abundant ; but as there are no 

 good exposures within easy range of Bath, it is not 

 necessary to do more than mention them. 



Much the same may be said of the next great forma- 

 tion, that of the Silurian, a term given by Sir R. Mur- 

 chison to the rocks which range between the Cambrian 

 and Old Red Sandstone. He considered them to be 

 typically developed on the eastern borders of Wales, in 

 a region once occupied by the Silures, and hence the 

 name Silurian. They are to be seen in the neighbour- 

 hood of Tortworth, in Gloucestershire ; but the exposures 

 are not good, and would hardly repay a visit unless to 

 a specialist who had some particular object in seeing them. 

 May Hill, on the borders of Herefordshire and Gloucester- 

 shire, is a well-known locality for what Murchison, in 

 1834, termed the " May Hill Series," which include — 

 (3.) Tarannon shales. 

 (2.) Upper Llandovery rocks. 

 (1.) Lower Llandovery rocks. 

 The Silurian period was brought to a close by conditions 

 which allowed of the deposition of a series of sandstones 

 and shales known as the Dawnton Sandstone, but which 

 are really passage-beds from the Silurian to the Old 

 Red Sandstone. At Ledbury, between Gloucester and 

 Malvern, these interesting beds have been cut through 

 by the railway to Malvern, and the section thus exposed 

 is one of the finest known. Mr. Piper, F.G.S., a local 

 resident, has made a special study of this strata, and has 

 obtained a good collection of the fossils which occur; 

 among them are Cephalaspidean fish belonging to the 

 genus Auchenaspis Egertoni and A. Salteri. Though 

 Ledbury is located at the extreme north-western limits 

 of Gloucestershire, it is easily accessible from Bath. 

 Quick trains on the Midland Railway run to Gloucester, 

 where it is necessary to change on to the new line to 

 Malvern. On arriving at Ledbury the passage-beds are 

 seen close to the station. For geologists who have not 

 had an opportunity of previously examining the strata a 

 journey is well worth undertaking from Bath. 



In the Mendip area of Somersetshire, and in Glouces- 

 tershire and Herefordshire, the Devonian period, which 

 follows the Silurian, is represented by the Old Red 

 Sandstone and Conglomerate, but in Devonshire and 

 West Somerset the strata of this age is all termed 

 Devonian. One locality, where the Old Red can be 

 studied, is below the gorge of the Avon at Clifton, a 

 suburb of Bristol. There a continuous succession of 

 Old Red and the rocks of the succeeding carboniferous 

 period may be studied without a break. It is most 



probable that an excursion will be made to this pic- 

 turesque and interesting spot during the meeting of the 

 British Association at Bath, and therefore it will be well 

 to refer to it in some detail. 



Commencing, then, with the Old Red Sandstone, we 

 find it represented by two types of rock — namely, (1) 

 red and mottled sandstones, and (2) by a conglomerate. 

 The former of these are more or less continuous from 

 Portishead to " Cook's Folly," below Durdham 

 Downs. The life of the period was remarkable for the 

 fish which lived in the waters; and so characteristic are 

 they of Devonian days that the period has appropriately 

 been popularly termed the "Age of Fishes." They mostly 

 belonged to the Ganoid and P/aeoz'a? groups, more especially 

 to the former. The Ganoids were fish having angular scales 

 regularly arranged, composed of horny or bony plates, and 

 covered with a strong, shining enamel. This group of 

 fishes is now nearly extinct, the best known remaining 

 representatives being the Lepidosteus (bony pike) of 

 America, the Polypterus of the rivers of Africa and 

 the Nile, and the sturgeons. At Portishead a fish- 

 bed exists which has yielded Holoptychins and 

 Coccosteus. At the foot of "Cook's Folly" the upper- 

 most beds of the Old Red period are represented. They 

 are those of the conglomerate, which is made up of 

 opaque quartz pebbles, called by Sir H. de la Beche 

 " veinstone quartz," and grains of sand, the whole being 

 cemented together by a siliceous ferruginous matrix. 

 Following the Conglomerates in the order of succession, 

 we find the pebbles become less numerous, and finally 

 almost disappear. The strata then becomes of a loose, 

 sandy nature with some shales, which indicate that the 

 conditions under which the Conglomerate was formed 

 were gradually coming to an end, and that another period 

 of the history of our planet was about to begin. That 

 period was the Carboniferous, during which great thick- 

 nesses of limestone were first formed. Then an alteration 

 took place, and vast quantities of sediment were spread 

 over the sea floor, in much the same way that 

 sandy debris is brought down into the open sea by the 

 river Plate in South America. Apparently the limestone 

 was deposited in the open sea, and was brought to an end 

 by the rising of the sea floor, which allowed of the forma- 

 tion of the Millstone Grit from the debris derived from 

 the denudation of the land. But during that time the 

 sea-floor continued to rise, and after over 900 feet of 

 sediment (Millstone Grit) had been accumulated, the 

 land surfaces appeared on which vegetation grew, the 

 carbonised remains of which we call coal. 



But we must study the Carboniferous rocks more in 

 detail. In Gloucestershire and Somersetshire they may 

 be divided as follows : — 



Greatest Thicknesses. 



Coal Measures 





6,700 feet* 



Millstone Grit 





900 „ 



Upper Limestone 





100 „ 



Middle Limestone 





1,600 „ 



Lower Limestone 



Shales 



99° » 



Above the sandy beds which mark the close of the 

 Old Red Conglomerate, we find a mixture of dark lime- 

 stones and shales, hence the name Lower Limestone 

 Shales. To understand the history of these limestones 

 they require to be reduced to thin sections, and be 



* This represents the thickness of the Coal-measures in Bristol. 

 In the Somersetshire Coalfield the thickness is a little over 

 7,500 feet. 



