ON THE 8T0CKDALE SHALES. . 715 



The apparent unconformity by which one of us was formerly de- 

 ceived turns out, on examination, to be due to the existence of strike- 

 faults. Although there is absolute conformity between the lowest 

 beds of the Stockdale Shales and the highest beds of the Ashgill 

 Shales, the palaeontological break is complete, and it is at this point 

 that we draw the line of division between the Ordovician and 

 Silurian systems. 



Similar conformity is seen between the top of the Stockdale-Shale 

 series and the base of the succeeding Coniston Plags, the passage 

 being rather a gradual one, however, instead of very sudden, as in 

 the case of the junction at the base. 



In all the sections described, with the possible exception of that 

 at Spengill, the Lower Skelgill Beds are seen to be separated from 

 the higher beds of the group by a strike-fault ; and minor faults of a 

 similar nature are seen at higher levels. That this fault does not 

 remove any great thickness of rock is shown : — 



(1) By the frequent juxtaposition of the zones of Dimorjjhograp- 

 tus confertus, and Monograptus Jimhriatus, which contain several 

 species in common. 



(2) More particularly by the resemblances between the above- 

 named zones and two similar zones in the Moffat area, the zones of 

 Diplograptus vesiculosus^ Nich., and Monograptus gregarius, Lapw., 

 and the relationship of the M.-fimhriatus zone to the lower portion of 

 the latter. As the succession is complete in the Scotch region, no 

 important zone of rock can be concealed in that of the Lake-dis- 

 trict. 



The occurrence of a fault of such wide extent exerting so little 

 effect is remarkable, and we can offer proofs that the fault itself is 

 not an ordinary one, but that it runs generally with the bedding, 

 and that it is rather of the nature of a crush. The soft SkelgiJI Beds 

 lie between the harder rocks at the summit of the Ordovician series 

 and those of the Browgill group, and they must have given way 

 during a process of stretching, which caused the upper beds adhering 

 to the Browgill rocks to move over the lower beds, which have 

 been shown to adhere to the Ashgill Shales. That such stretching 

 has taken place is proved by the following facts : — 



(1) The behaviour of the strike-fault with the beds. The out- 

 crop of the Skelgill Beds is a sinuous one, being bent into Y's in 

 crossing the valleys, the apex of the Y's always pointing down the 

 valleys, as the dip of the beds is always greater than the slopes of 

 the valley-bottoms. Moreover, these valleys are usually occupied by 

 great dip-faults, which disj)lace the beds laterally. In such cases 

 the strike-fault would be found to affect different beds when crossing 

 these valleys and shifted by the dip-faults, whereas it is always found 

 to run along the narrow band between the Ashgill Shales and the 

 Browgill Beds. 



(2) The Skelgill Beds are often entirely, or almost entirely, 

 removed. When developed in force, there is usually a section cut 

 through them, and the narrow depressions occurring between the 

 different sections and marked by a line of swamp, are bounded by 



