220 Br. C. Callaway — On Plagloclinal Mountains. 



This series has been named the Dimetian. It is flanked on both 

 sides by a younger Precambrian formation, the Pebidian. In 

 Dr. Hicks' sections in the Quart. Journ. of the Geol. Soc. 1877 and 

 1878, the junction between the two groups is not represented by a 

 fault ; but it is difficult to explain the vertical and sometimes 

 inverted position of the lower beds of the Pebidian without the aid 

 of such an hypothesis, and a recent inspection by the writer con- 

 firms this view. But this is only a minor point. In any case, the 

 Dimetian ridge is on the type of the Shropshire and Malvern chains. 



The same peculiarity has been noticed in North Wales. Professor 

 T. M 'Kenny Hughes, in describing certain Precambrian rocks near 

 Bangor, observes that " the bedding is generally in an easterly 

 direction, sometimes a little S., sometimes a little N. of E., but 

 always oblique to or across the trend of the hills." No information 

 is given on the influence of faults in determining the structure of 

 these ridges. 



In summing up these facts, it has been shown that, in the three cases 

 examined by the writer, the trend of the elevations has been deter- 

 mined by parallel faulting ; in the other example, the influence 

 of faults has not been ascertained ; in all four, the strike of the 

 beds is transverse to the strike of the ridge. 



On the hypothesis of parallel faulting, it is not difficult to suggest 

 a reason why these transverse strikes should be found only in 

 Precambrian mountains. In every observed case in Britain, there is 

 a great interval between the newest Precambrian and the oldest 

 Cambrian. The marked discordance of strike is one proof of this. 

 An equally emphatic evidence is seen in lithological characters. 

 The Precambrian rocks are everywhere intensely altered, and the 

 alteration in many instances took place before the Cambrian epoch. 

 But the overlying Cambrian strata are frequently as unaltered as 

 ordinary Tertiary beds. It is highly probable that the fragments of 

 Precambrian land which jut up here and there through newer for- 

 mations represent a succession of epochs perhaps as long as from the 

 Cambrian to the present day. Paleeontological facts point in the 

 same direction. The gap between the simple Protozoan of the 

 Laurentian and the highly organized Trilobites of the Lower Cam- 

 brian is perhaps as wide as the hiatus which stretches between the 

 Trilobite and the Mammal. 



It follows from this that Precambrian formations have been sliced 

 up by faults much more frequently than younger deposits. It is not 

 necessary to assume, though the assumption would not be rash, that 

 faults were more frequent in Precambrian times than they are at the 

 present day ; but it is evident that the Precambrian crust was 

 traversed by Precambrian faults in addition to those from which it 

 has suffered in common with newer deposits. 



In Precambrian times, the surface of the earth was ridged by 

 mountains as at present, and it is fair to assume that they were 

 formed on the normal plan. But during an immense succession of 

 epochs, they have been partially worn down, and the chief part of 

 the Precambrian surface has been buried beneath great accumula- 



