THE STRUCTURE AND CLASSIFICATION OF KOCKS. 



639 



This synopsis 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, 



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



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

 district, or quite naked to inspection, 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. 



The first nine of the systems named contain, in a varying degree, the remains of plants 

 and animals. The other three exhibit no traces of them, except the upper part of the 

 clay-slate system, in which they are occasionally found. Upon comparing the remains 

 exhibited by formations of different ages together, decisive evidence appears of the pro 

 gressive development of organic life, from the more simple and imperfect structures that 

 obtain in the older strata, to the higher organisations that are found in those depositions 

 which belong to seras immediately antecedent to the existing epoch. In the course of our 

 planet's history, the less complex tribes of animals and plants were the first to appear, the 

 more perfect species becoming more and more numerous up to the creation of the present 

 races. This is not generally held by geologists as inferring a gradual perfectionnement 

 of species. It is true that fishes, which of all vertebrata rank the lowest, appear 

 first in geological history ; but then they are not imperfect formations they 

 have no mark of inferior organisation, but occur in their highest state of 

 approximation to the reptile, and not in their lowest condition of affinity to the 

 worm. It is also true, that reptiles precede mammalia, but they are reptiles 

 belonging to the higher grades of that class. The earliest zoophytes and mollusca likewise 

 display no inferiority in their organisation when compared with their living representatives. 

 Hence it is concluded, that while there has been a progressive development of 

 organic life upon the face of the globe, it has not been by an improvement of species, but 

 by an addition of fresh organisations, a new dramatis personce, to meet new physical con 

 ditions of the earth. 



