78 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [May 1894, 



constructed by Mr. Ussher l the whole of the great triangle which 

 constitutes the southern lobe of Devon is coloured as Lower 

 Devonian. Apart from the somewhat fanciful idea of including the 

 Start rocks in this category, it is very much what we should expect 

 from indications already made known before Mr. Ussher had seen 

 tit to change his mind on the subject of the Cockington Grits. The 

 great development of Lower Devonian beds in this maritime area, 

 continued in a westerly direction through Looe and thence right 

 across the heart of Cornwall, adds to the completeness of the broad 

 synclinal which is the leading physical feature of the south-western 

 peninsula. 



The Older Paleozoic Rocks. 



Silurian and Ordovician. — I presume that it is right to include 

 the Arenig series with the Ordovician, these two systems or sub- 

 systems thus constituting the upper division of the Older Palaeozoic. 

 We have had about half a dozen papers in this category. Of these, 

 two by Messrs. Marr and Nicholson have reference to the North- 

 west of England ; there is a stratigraphical paper on the Llandovery 

 rocks of the neighbourhood of Corwen by Messrs. Lake and Groom, 

 and papers on special subjects by Prof. Rupert Jones and 

 Mr. Wethered. There is also a note on the geology of the district 

 west of Caermarthen from the pen of the late Thos. Roberts, 

 wherein he records the discovery of the Tetragraptus-beds of 

 Arenig age, which had not hitherto been detected south of the 

 St. David's district. 



Messrs. Marr and Nicholson's first paper relates to the Stockdale 

 Shales, which extend across the main part of the southern half of 

 the Lake District, parallel with the underlying Coniston Limestone 

 series and the overlying Coniston Plags, with both of which they are 

 conformable ; i. e. they are conformable to the Ordovician beds below 

 and to the Wenlock beds above, thus representing the two Llandovery 

 subdivisions and the Tarannon Shales of the Welsh Border area. 

 The Authors also correlate the Graptolite-zones with those of the 

 Birkhill and Gala groups in the South of Scotland. Although the 

 whole group, in the area examined, attains to no more than 400 feet 

 as a maximum thickness, the Authors indicate something like 

 seventeen zones or horizons, many of them distinguished by a 

 particular graptolite. These Stockdale Shales are regarded as being 

 divisible into a Lower group, viz., the Skelgill Beds, consisting 



1 Prop. Somerset. Archceol. Soe. toI. xxxviii. 1892. 





