1 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 



a striking resemblance between the Bala and the South Dee series, 

 modified occasionally by an irruption of felspathic rocks amongst 

 the latter. The Cambrian slaty rocks without fossils form, as Mr. 

 Sharpe states, an irregular saddle in the middle of the Berwyns ; and 

 the anticlinal axis of the South Dee Silurian and Cambrian rocks 

 extends to Cader Berwyn, the summit of which is a partially columnar 

 greenstone. Again, on the Holyhead road Lower Silurian schists 

 are found, covered up by unconformable Wenlock strata. On the 

 road from Caernarvon to Holyhead the same junction of the Wen- 

 lock and Lower Silurian is observed ; but we shall not further follow 

 this part of the subject, as Mr. Sharpe appears fully to have esta- 

 blished the fact that all the fossiliferous beds are characterized by 

 true Silurian fossils, according to the most generally recognized 

 meaning of the term Lower Silurian. The descriptions of the igne- 

 ous rocks and the explanation of the condition of the slates worked 

 west of Snowdon, which are shown to be a repetition of the same 

 bed produced by contortions consequent on the intrusion of green- 

 stone, the determination of the various axes of disturbance, faults, 

 &c., are all proofs of close and careful observation. Mr. Sharpe 

 makes also some very judicious remarks on the diiferences observable 

 between the Lower Silurian and Cambrian rocks of North Wales and 

 those of the English Border Counties. 



The comparative paucity of organic remains he considers to de- 

 pend on the greater thickness of the beds, or on the greater sea- 

 depth of the deposit, in conformity with the principle established 

 by Edward Forbes, — that the number of species and of individuals 

 decreases as the depth increases, but that the lateral range of species 

 increases as the depth increases, so that in deep-sea deposits of the 

 same age there should be fewer species and a greater similarity be- 

 tween them at different localities. The Wenlock series near Llangollen 

 is 3500 feet thick, — a vast accumulation compared with that of the 

 same series in Worcestershire ; but in addition to this evidence of a 

 deep-sea deposit, Mr. Sharpe adduces also the ordinary habitat of 

 the genus Creseis, which in the Mediterranean abounds in the muddy 

 bottom of the sea at great depths, and is only sparingly found in 

 more shallow water ; and I must consider the natural-history proof 

 the best in such cases, as it is so difficult to determine what physical 

 changes may have been taking place during the deposition of such 

 vast masses. 



Mr. Sharpe first entered on the subject of cleavage in thispaper; and, 

 following up the law estabhshed by Professor Sedgwick, that cleavage 

 planes maintain their parallelism over extensive areas, irrespective of 

 the varying position or the mineral character of the beds they cut 

 through, he arrived at the further law, that the strike of the clea- 

 vage coincides with the prevailing or normal strike of the beds, and 

 continues uniform, not varying with the local variations of the strike 

 of the strata ; thence concluding that cleavage cannot be the result 

 of crystalhzation : and again, as the planes are arranged with such regu- 

 larity, and are not affected by the variations in the strike of the strata, 

 that they are not the effect of mechanical force or pressure exerted 



