642 GEOLOGY. 



This diagram exhibits a view of the succession of strata as developed in Great Britain, 

 and, excepting some circumscribed continental tertiaries, we have, within our own island, 

 the representative or the equivalent of almost every formation whose existence has been 

 ascertained. This order of succession is invariably maintained ; for though many members 

 of a group may be wanting, and also many groups, those that occur are found universally 

 to occupy the same relative position. We may suppose the series to occur, 



A A A AC 



B B B D B 



C C - - - 



D or as or as but it never occurs as B or as F or in any such inverted order. 



O G O F E 



Thus, a system low in the diagram may be found immediately under the surface soil of a 

 district, or quite naked to inspectfon, all the intermediate groups being absent ; as the 

 magnesian limestone in Durham, the coal in Fifeshire, the old red sandstone in Here- 

 fordshire, and the slate in North Wales ; but if these lower groups in the diagram appear at 

 the surface, then, however deeply we might pierce the strata there, we should never come 

 to any of the systems higher in the scale. It would be vain to sink a shaft in search for 

 chalk in Durham, or for magnesian limestone in Fife, or for coal in Hereford, or for 

 the old red sandstone in the neighbourhood of Plinlimmon, because the formations sought 

 are superior in the series to those which occupy the surface in the localities named. 

 Hence a practical knowledge of the succession of strata would have prevented the expend- 

 iture of thousands in the search for coal, in situations geologically beneath it, or so far 

 overlying it as not to be accessible. The absence of groups of strata, to which we have 

 referred, may have arisen from the action of water, subsequent to deposition, denuding 

 and washing them away, diffusing their materials upon the floor of the ocean, or entering 

 into fresh formations ; or other modifying circumstances may have prevented their deposi- 

 tion altogether in such sites. 



Travelling from the metropolis across our own island in a north-west direction^ we 

 successively meet at the surface with the several groups of strata which appear in the 

 diagram. Around London we find the tertiary deposits, which spread from thence over 

 nearly the whole of the eastern coast of England to beyond the Humber, covered with 

 accumulations of alluvium and drift. The chalk is encountered in the counties of Hertford, 

 Bedford, or Buckingham ; the oolite in Northamptonshire ; the new red sandstone and 

 carboniferous systems in Leicestershire and Staffordshire ; the old red sandstones and 

 Silurians in Salop ; and lastly, the Cambrian, mica-schist, and gneiss groups occur in the 

 alpine or mountainous districts of Great Britain, which extend, with some interruptions, 

 along its western side. In describing the several systems of strata, some writers traverse 

 the series descendingly, commencing with the uppermost or most recent deposits ; but the 

 more natural plan is to proceed ascendingly, beginning with the lowest formations, the 

 earliest in point of time) and the storehouse whose materials have contributed to the 

 production of the rest. The first step in an arrangement of this kind involves a reference 

 to formations of more ancient date than the oldest of the strata the unstratified 

 foundation rock the granitic platform, upon which all the other deposits are based> sinking 

 to depths which the eye of man can never explore, and rising in mountain walls and 

 pinnacles to heights which only a few of his race have reached. This will form the 

 subject of the following Chapter, in connection with the other igneous masses in general^ 

 except those which have been formed during the age of man, which will hereafter be 

 noticed. 



