﻿Vol. 6 I.] OEDEB OF SUCCESSION OF THE MANX SLATES. 359 



concealed by the abundance of their own debris, which may be more 

 or less mixed and removed from their original site ; and, in the 

 second place, to the similarity in many of their aspects of portions 

 of the different members themselves, when seen otherwise than in 

 mass. And when, to these difficulties, we add the easy confusion so 

 often introduced by cleavage of a complicated character, and the 

 indubitable contortion to which parts are subject, we have all the 

 elements united which might lead us to despair of ever solving 

 the problem. 



There is no cause for wonder, then, that we read such expressions 

 as the following in Mr. Lamplugh's recent Survey-memoir : — 



' It cannot be pretended that more has been done than to trace out the 

 broader elements of their stratigraphy. . . . There are broad tracts ... in 

 which the different varieties are so closely intermingled as to be practically 

 inseparable. . . . Their relative order has not been established with any 

 certainty, and still remains a matter of inference. . . . The chief impediment 

 to more detailed stratigraphical work has been the failure to find bands 

 smaller than the[se] main divisions, with characters distinct enough to be 

 constantly recognizable. . . . The apparent dips are altogether misleading as 

 to the superposition and stratigraphical relations of the strata.' ( Op. cit. pp. 28, 

 29,30.) 



Phrases such as these, together with the adoption of no colour for 

 a wide area left as ' unseparated/ show that the order of succession 

 has not been determined ; and we are left without the knowledge 

 whether the fragments contained in some of the rocks called con- 

 glomerates are derived as usual from the strata below them, or, by 

 exception, from the strata above them ! 



In determining the order of succession in a series of unfossiliferous 

 strata, we are deprived of the direct guidance of recognizable fossils, 

 and have consequently to depend on stratigraphical considerations 

 which have been less studied but which, from experience, render it 

 more probable that fine-grained deposits follow coarser-grained, 

 except for ' episodes ' ; and show that the more metamorphosed rocks 

 are usually cceteris paribus the older. Details of structure, such as 

 cleavages of various kinds, do not affect the order of succession, and 

 may therefore be neglected, for a rock when uncleaved will have the 

 same position relatively to its neighbours as when cleaved. 1 These 

 and many similar axioms render it unnecessary to determine the 

 nature of every rock if, by the selection of suitable spots, we can 

 determine the order of succession in parts, and unite the observations 

 into a whole. 



(1) The Barrule Slates. 



We commence with the rocks which form the central axis of 

 the island, and rise to the greatest height. They are called the 

 Barrule Slates by Mr. # Lamplugh, and are of great visible 

 thickness. They are black and characterless where most fully 

 developed, the only apparent divisional planes being due to cleavage. 2 



1 Further details with regard to ' nappes de charriage ' are requii'ed ; see 

 Marcel Bertrand, Bull. Soc. Geol. France, ser. 3, vol. xxvi (1898) p. 632. 



2 By this term it is intended to include all the smaller changes commonly 

 assigned to ' earth-movement.' 



2c2 



