THE BASAL ROCK-(J ROUPS OF SHKOPSHIKK. 119 



and I thought it convenient in 1887 * to give to this great group 

 the local designation of " Longmyndian." Prof. Lapworth has 

 since announced the discovery of fossil evidence for tlie Lower- 

 Cambrian age of the Hollybush Sandstone ; but, pending the pub- 

 lication of his details, it would hardly be wise to positively assign 

 a pre-Cambrian age to the Longmyndian system. 



Our present object is to ascertain if there is a time-break between 

 the Uriconian and Longmyndian, and, if so, whether it is a small 

 or a great one. 



That there is a break will, I think, appear from the following 

 considerations. 



1. The Discordance op Strike between the two Groups. 



It is well known that the normal strike of the Longmynd rocks 

 is N.X.E. I have made observations in hundreds of localities, from 

 Haughmond Hill on the north to a newly discovered iulier on the 

 borders of Herefordshire on the south ; and I have found that, 

 though there are occasionally slight deviations and abrupt twists, 

 the normal strike, even in faulted masses, is maintained with re- 

 markable uniformity. The strikes in the map which faces the next 

 page have all been personally verified. 



On the other hand, as I pointed out in 1879, the normal strike of 

 the Uriconian is more or less transverse to the above. In a volcanic 

 series, we should not expect the strikes to be uniformly parallel or 

 persistent for great distances ; nevertheless, it will be found that 

 the exceptions to the above rule are not very numerous. I have 

 recently reviewed the original evidence with great care, and have 

 discovered new strikes in all the chief masses. The enlarged evi- " 

 dence is summarized in the map, which represents the minimum 

 number of strikes observed, but where several occurred in the same 

 vicinity only one is usually drawn. I have been careful to supply 

 nothing from the imagination. Even when a curve in the strike 

 appeared to be fairly dedueible from several disjointed exposures, I 

 have not connected the broken lines, unless they were so near 

 together that it would be mere pedantry to refuse to join them. I 

 will now take the chief areas in order. 



(a) Lillesliall Hill. — Strikes are clearly seen almost from end to 

 end of the ridge. Some of them are in rhyolite, others in slaty 

 ash-beds. They vary between E. and W., and j^.E. by E. 



(b) The Wrekin Chain. — A strike, originally mapped as occurring 

 in the so-called " granitoidite " of the Ercal, must be erased. The 

 rock is novv regarded as a true granite, and its band of fragments 

 is probably a crush-breccia. The transversal strikes in the rhyolite 

 at the southern end of the Ercal, as also those in the hornstones, 

 ashes, and agglomerates of Lawrence Hill and the northern end of 

 the Wrekin, are confirmed. I have moreover made out a number of 

 new strikes in rhyolite along the jST.W. side of the Wrekin nearly as 

 far south as the summit, and I find that most of them trend E.X.E. ; 



* Trans. Shropsh. Archgeol. Soc. for 1887. 



