350 A Ramble in West Shrojishire. 



process must at one time have gone forward. Between 

 the deposition of the Lingula flags of the Stiperstones, 

 and the Llandeilo, which overlie them so regularly that 

 there is nothing in this neighbourhood to suggest that 

 they are not one stratum, there is believed to have been 

 au enormous interval of time, since their fossils are widely dif* 

 ferent. Well, in North Wales are found strata which, judged 

 by their fossil contents, are the equivalents of those of the 

 Stiperstone rocks, and again strata equivalent to the Llandeilo ; 

 but between them a huge slice, called the Tremadoc slates, with 

 fossils of a character intermediate between those of the strata 

 above and below it. It is, therefore, to be presumed that this 

 slice is the representative of a vast cycle of chang-e in the 

 organic life of these regions, of which no vestige whatever is 

 left in the neighbourhood we are now examining: — a missing* 

 link in the development of organic life is found, and a prob- 

 lem solved which would otherwise be inexplicable. But, still 

 farther, we are taught the important lesson of caution in our 

 deductions from observed phenomena. It might have been 

 inferred, from the sudden change in the organic life displayed 

 by these successive strata, that its progress was capriciously 

 interrupted and its character suddenly changed at a certain 

 epoch ; but this discovery of the Tremadoc slates with their inter- 

 mediate fossils, is an additional reason to believe in that gradual, 

 progressive development of species which so many other lines 

 of research would seem to affirm • and it, moreover, shows that 

 when, in studying' other parts of the great geological volume, 

 we might occasionally be led to suppose exceptions to this 

 great law, such a conclusion may arise from some other missing 

 links, some other pages, or even entire chapters, having been 

 torn rudely from the venerable record. 



At last we are prepared to descend through one of the 

 numerous gorges, dingles, or cwrns (as they are sometimes locally 

 called) which penetrate these strata, running into them from 

 the west, and exposing the Llandeilo rock, dipping at a very 

 high angle towards the same quarter. - Their thickness has 

 thus been estimated at not less than 3000 feet. For the most 

 part they are barren of fossils, except at the very top or the 

 very bottom of the beds ; to use the familiar illustration of my 

 friend Mr. Salter, like a thick piece of bread well buttered on 

 both sides. It will be understood that the bottom strata con- 

 stitute the high hills which we have just left, and the higher 

 are found in the valleys upon which we are now entering. As 

 I stated before, the only spots in which I am aware of fossils 

 being found are the little hollows on the high grounds formed 

 by the sheep for shelter. There are other places mentioned in Sir 

 R. Murchison's Siluria, such as Lord's Hill, near the Chapnl; 



