290 Reports and Proceedings. 



their course, and leaving behind them an exhausted area ; this 

 motion has sometimes taken place in the plain of stratification. 

 Occasionally centrifugally advancing from a depleted area, more 

 frequently centripetally towards a nucleus which it ultimately 

 environs with a ferruginous crust. 



11th. The bleaching of the Cambrian slates is due to two distinct 

 causes, viz., (a) the occurrence of light bands adjacent to inter- 

 bedded dark green layers, to the actual departure of the greater paii; 

 of the colouring oxide without any increase in the proportion of 

 protoxide to peroxide, and appears analogous to the local bleaching 

 of red beds, (h) The conversion of blue and purple slate to green 

 in large fields of colour by the conversion of most of the peroxide of 

 iron to protoxide, and to which the green discolouration adjacent to 

 the dykes of intrusive diabase is analogous. 



2. "On the older Eocks of South Devon and East Cornwall." 

 By Harvey B. Holl, M.D., F.G.S. 



The author divided the rocks of the district to which the com- 

 munication referred into a Lower, Middle, and Upper South Devon 

 Group, and stated that the lowest beds were brought up along a 

 line of country extending from Dartmoor by Hingston Down to the 

 Brown Willey granite, where they formed a broad anticlinal axis. 

 These rocks are unfossiliferous, and may not be lower in the series 

 than the base of the Ilfracombe group of North Devon, or the highest 

 part of the group immediately below it, the latter being more pro- 

 bably represented by some still lower beds of red and greenish grits 

 brought up to the surface in the anticlinal axis of St. Breock's Down 

 further to the west. 



The Middle South Devon Group comprises at its base the discon- 

 tinuous calcareous range of the Looe Eiver, St. Germans, Brickfort- 

 leigh, Ashburton, and Bickerton, above which is a mass of blue and 

 claret-coloured slates, which separates it from the upper or Plymouth 

 and Torbay Range. 



This calcareous and fossiliferous group is succeeded by higher beds 

 of blue and claret-coloured argillaceous slates, followed by hard, red, 

 micaceous schists, and purple and greenish grits, which constitute 

 the author's Upper South Devon Group. These rocks are very 

 sparingly fossiliferous, and probably correspond to the upper and 

 Morthoe portions of the Ilfracombe series of North Devon. 



The uncomformable position of the Culm-measures is seen in the 

 circumstance that they rest upon different parts of the underlying 

 Devonian rocks ; sometimes on the limestones of the Torbay Range, 

 sometimes on the slates, at others on the volcanic rocks. This un- 

 conformability entirely separates the older rocks of South Devon 

 from the Carboniferous System. 



The occurrence of the genus JPteraspis and probably Cephalaspis, 

 with Phyllolepis concentricus and (?) Holoptychius, and other fish- 

 remains, appeared to the author to go a good way towards identify- 

 ing these Cornish and South Devon beds with the Old Red Sandstone 

 of Scotland. These fossils range up to the very base of the Torbay 

 limestones. 



