314 Dr Murray on a Deposite of Fossil Plants, 



followed by difference in petrifaction, even under similar cir- 

 cumstances, we have innumerable instances. The common le- 

 pidodendrons of every coal measure are completely changed, as 

 to the main trunk, into sandstone ; while the cortical envelope is 

 only carbonized. The soft succulent interior is wholly gone, 

 and its place supplied by the stony matter of the investing 

 strata ; but the firmer harder bark still exhibits traces of its ori- 

 gin, sufficiently distinct to designate the natural order, if not 

 even the very genus. 



In the immediate vicinity of these fossil plants, much inter- 

 ruption, and consequent ambiguity, takes place in the regular 

 arrangement of the strata, by frequent and extensive slides or 

 slips of the rocky beds ; so that, to any casual observer, the 

 higher line seems occasionally to lie beneath one, in reality, far 

 its inferior in order ; and this especially happens, whenever a 

 slide of the gravelly diluvium has descended, so as also to cover 

 the intermediate deposites. And, in similar instances, where any 

 resemblance exists between the upper and lower strata, the con- 

 fusion will be yet greater, and will demand no little strictness of 

 investigation. 



Even stratifications similar in relative position, and in proba- 

 ble antiquity, and undisturbed in their situations, vary so very 

 much in colour, structure, and chemical composition, as to de- 

 fy all classification by any sensible qualities ; while their precise 

 position is oftentimes so perplexed and obscured, as to render 

 the difficulty not removeable by that resource. 



But here it is, that the vast excellency and usefulness are 

 shewn of the plan laid down by M. Brongniart, in France, and 

 Mr William Smith, in England, who shew that similar fossils 

 characterise similar formations, and thus give us the means of 

 determining the nature and place of any strata. By noting the 

 contained petrifactions, an observer may thus readily pronounce 

 whether a mass, however displaced, belong to the highest oolite 

 or the lowest, or to the undecomposed marlstone. Another in- 

 stance in point may be cited in the green sand, so frequently, if 

 we may be allowed so to speak, of all colours but green, and 

 varying also exceedingly in structure, yet is well and decidedly 

 marked by its numerous and beautiful fossil shells. 



It is one of the many advantages of geological knowledge, 



