72 



THE POPULAK EDUCATOR. 



definition and the prominence of its position. It can never be 



mistaken. 



The sandstones of the lower carboniferous reach a noted de- 

 velopment in Ireland, and may be reckoned hardly second in im- 

 portance to the arenaceous accumulations of the preceding 

 period, the old red sandstones. While the deposits of coal 

 are so noted that they not only have no equal, but not an 

 approach to a rival, these three/ important features of this 

 period together make the carboniferous the most celebrated of 

 all the systems. 



In England the system is developed in many places, but the 

 main axis is a broad rectangular patch which stretches from 

 the border of Scotland to the town of Derby; it occupies 

 the whole breadth of the island, as far south as Whitehaven 

 on one side, and Hartlepool on the other. In both cases the 

 line then leaves the coast, still preserving a more or less perpen- 

 dicular direction, till on one side it reaches the neighbourhood 

 of Crewe, and on the other the town of Nottingham ; then a 

 line joining these places and 

 passing through Derby will form 

 the southern boundary of tho 

 axis. A glance at a geological 

 map will show all the places 

 where the members of the sys- 

 tem appear, far more accurately 

 than we can describe them. 



The most convenient division 

 of the strata which lie between 

 the old red sandstone beneath 

 and the new red sandstone 

 above, seems to be - 



1. Lower Coal Measures. 



2. Mountain Limestone. 



3. Millstone Grit. 



4. Upper or True Coal Measures. 



1. The lower coal measures 

 include all the strata which 

 appear between the old red sand- 

 stone and {(he mountain lime- 

 stone ; they are but scantily 

 developed in England, but ap- 

 pear in full force in the south 

 of Ireland, and also in Scotland. 

 They bear a subdivision into 



1. Carboniferous Slates. 



2. Yellow Sandstones. 



The yellow sandstones, some- 

 times named after the Irish geo- 

 logist, Sir E. Griffiths, Griffiths' 

 sandstones, rest nnconformably 

 on the old red on the south- 

 west coast of Ireland, and are 

 again and again exhibited to 

 the very north of the island. 



These yellow sandstones vary in their thickness from 400 

 to 2,000 feet, and it is but right to state that their position is 

 not yet finally established ; there is much reason for believing 

 them to be co-ordinates of the Dura Den sandstones, a group 

 certainly Devonian, which are developed in Fifeshire, and 

 contain fossils of the Holoptychius and Pterichthys, which are 

 eminently creatures of that period. Yet, until this is firmly 

 established, we prefer to leave them where they are generally 

 inserted, as the lower members of the carboniferous group ; 

 and indeed there is much direct reasoning in favour ci this 

 position ; for although in them, as in the rest of the sandstones, 

 fossils are few and far between, yet those which do occur 

 possess strong carboniferous relations : for example, specimens 

 of the Lepidodendron, Calamitis, Stigmaria, and Cyclopteris 

 have been discovered, and, as we shall find, these are the main 

 species which the woods of the carboniferous world contained. 



These yellow sandstones are found in England in Gloucester- 

 shire, and in South Wales. 



Carboniferous Slates. Tho upper division of the lower coal 

 measures is divided by Professor Jukes into 



3. Coomhola Grit Series. 



2. -Dark Gr.;y nncl Black Slates and Shales. 



1, Lcvrer Limestone Shale with calcareous bauds. 



The Coomhola Grits are yellow sandstones into which are 

 inserted grey and black shales ; they are found in the im- 

 mediate neighbourhood of Bsntry Bay, and are but scantily 

 fossiliferous. 



The Dark Grey and Black Sates and Shales are very similar 

 to the series which lies beneath them, and are yet a formation 

 which was deposited under somewhat altered circumstances ; 

 for they contain no slates, which marks an absence of deep 

 water where the argillaceous sediment quietly reached the 

 bottom. 



The Lower Limestone Shale is, in reality, tho transition bed 

 between tho lower coal measures and the mountain limestone. 

 In this bed the calcareous bands begin to appear, and, on the 

 other hand, in the great deposit immediately above it all sand- 

 stones disappear, and the calcareous matter gains the pre- 

 dominance. 



This series begins to bear evidence of the fossiliferous 

 character of the mountain limestone, for in the calcareous 



bands which traverse it, an 

 abundance of marine fossils are 

 discovered. 



The lower coal measures in 

 Scotland, though not so typi- 

 cally or so fully developed, yet 

 roach a considerable thickness 

 in the districts of Fife and 

 Lothian ; they have none of the 

 slaty character of their Irish 

 relations, but are mainly white 

 sandstones, interstratified with 

 bands of dark bituminous shales. 

 As in Ireland, these shales owe 

 their dark colour to the presence 

 of that vegetable matter which, 

 in the upper parts of the series, 

 accumulated to form seams of 

 coal. 



In Ireland it never exists in 

 a separate form, though in Scot- 

 land it appears in thin seams 

 of coal. The iron which tinged 

 the deposits of tho Devonian 

 period with red, seems to have 

 increased in quantity so as to 

 form thin bands of ironstone, 

 which, higher up in the system, 

 form rich deposits. 



The fossils which this series 

 contains, proclaim that the beds 

 have an estuary origin, partly 

 marine and partly fresh- water ; 

 and to this fact may be ascribed 

 those peculiarities which have 

 been judged sufficient to rank 

 these beds as a distinct group. 



All corals which are exclusively marine existences are con- 

 spicuously absent, although in the mountain limestone imme- 

 diately above they occupy a prominent position. The bands 

 of limestones, such as those at Kingsbarns, Fife, which some- 

 times are three feet in thickness, are composed entirely of 

 shells of bivalves of the mussel tribe ; and when we know the 

 habits of these mollusks make them love an estuary existence, 

 we have another fact confirmatory of the fresh- water neighbour- 

 hood in which the lower carboniferous was formed. Moreover, 

 fragments of land-shells, and some remains of small reptiles of 

 the frog tribe, show the proximity of land and the influx of 

 rivers. 



2. Mountain or Carboniferous Limestone. This deposit id 

 always dark-coloured and semi-crystalline ; from its hardness 

 it forms bluff and bold escarpments, which may be seen in 

 many parts of Yorkshire, Derbyshire, Westmoreland, Fife, and 

 Ireland. It has received its name "mountain" from the fact 

 that it frequently occupies the higher grounds of the car- 

 boniferous period. 



It occasionally is found in masses several hundred feet thick, 

 which are here and there traversed by bands of shale. These 

 'masses are split up into rectangular blocks by the joints and 

 backs to which we alluded in a foregoing lesson; but often 



