ANNIVERSARY ADDRESS OF THE PRESIDENT. xlix 



affected even the Old Red Sandstone, deserves es])ecial attention in 

 any discussion upon the unity of the Lower and Upper Silurians. 



1842. — Following up his inquiries, Mr. Sharpe next proceeds to 

 investigate the age of the Bala limestone. Here we enter on that 

 debateable ground which has been the subject of dispute ever since. 

 In the first days of the labours of Sedgwick and Murchison, Pro- 

 fessor Sedgwick thought he recognized an Upper and a Lower Fossi- 

 liferous System, below the Lower Silurian of Murchison, and this 

 view was at first adopted by Murchison. The Bala and Coniston 

 limestones were thus at first considered contemporaneous formations, 

 and placed together in the Upper Cambrian ; but as Mr. Marshall 

 had classed, by the evidence of fossils, the Coniston limestone with 

 the Caradoc, — a view adopted by Mr. Greenough, Professor Sedg- 

 wick afterwards separated these limestones, retaining the Bala lime- 

 stone in the Upper Cambrian, but abandoning the Coniston to the 

 Lower Silurian. The object of Mr. Sharpe was to prove that the 

 Bala and Coniston limestones both belong to the Silurian. 



Li 1843 he again went over this ground, and corrected some 

 minor points connected with these papers. 



March 6, 1844. — Geology of North Wales. — In this paper Mr. 

 Sharpe proposed to himself to determine the very important question, 

 whether any fossiliferous deposits exist below the Silurian formation. 

 The Llangollen district was the first examined, and Mr. Sharpe con- 

 curred with the late Mr. Bowman in allotting its rocks partly to the 

 Ludlow and partly to the Wenlock series, the micaceous and shaly 

 beds of the Ludlow portion not being here separated into two parts 

 by any representative of the Aymestry limestone, and the slates and 

 shales of the Wenlock portion being rich in the characteristic genus 

 Creseis, to which Mr. Sharpe added three new species. The Silu- 

 rian rocks are, near Corwen, overlaid by an outlier of mountain-lime- 

 stone. In one portion of the district the Upper Silurian rocks (the 

 Ludlow with a mountain-limestone cap) rest conformably on the 

 Lower Silurian, whilst in another the Wenlock beds rest unconform- 

 ably on the Lower Silurian roofing-slate. This conformability is 

 stated to be rare in North Wales ; but in a district disturbed by 

 faults, unconformability alone cannot be considered sufiicient evidence 

 for the separation of the Upper from the Lower Silurians. 



Mr. Sharpe reviews his former section of the Bala beds ; and, enu- 

 merating nine, he includes the upper seven, all of which, excepting 

 the uppermost, are fossiliferous, in the Lower Silurian ; but he allo- 

 cates the two lowest, which are not fossiliferous, to the Cambrian ; 

 though the absence of fossils, on which the negative evidence for 

 fixing the line of separation between the two formations depends, 

 can rarely be considered sufficient. 



The region north and south of the Dee is next examined, and a 

 dark roofing- slate without fossils is again found to rest upon loose 

 schists full of fossils, and including two beds of fossiliferous lime- 

 stone. The fossils are those of the Lower Silurian, and the rocks 

 themselves rest unconformably upon a poor roofing-slate, considered 

 by Mr. Sharpe to be the upper portion of the Cambrian. There is 



VOL, XIII. d 



