400 PKOCEEDI^TGS Or THE GEOLOGICAL SOCIETY. [April 22, 



striking points is the very small proportion of tlie numerous forms 

 of variegation that can be accounted for by the mere altered state 

 of combination of the iron in situ. 



The occasional conversion of the red anhydrous sesquioxide, or the 

 lower hydrates, into fully hydrous sesquioxide, the reduction of 

 sesquioxide to protoxide of iron in the production of green slates, 

 and the exceptional cases of the alteration of colour of red beds by 

 the decomposition of bisulphide of iron complete the list of colour- 

 alterations by simple chemical change. 



Even the agency of organic matter in inducing chemical changes 

 in the state of combination of the iron, will not in most cases 

 account for the bleaching — the segregational motion of the colour- 

 ing oxide, which is the ultimate cause of the variegation, being 

 supplemental to the simple chemical changes of combination. The 

 great majority of cases of variegation are independent of altered 

 combinations, and more often than otherwise seem to have been 

 induced by agencies not directly connected with chemical change. 

 The transference of the colouring oxide from one part of the stratum 

 to another has taken place by the simple mechanical agencies of 

 infiltration and dissolution, as well as by segregation ; but the latter, 

 above all other agencies, has played the largest part in the variegation 

 of ferruginous rocks. 



2. On the Older Eocks 0/ South Devon anrZ East Coknwall*. By 

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



[Plate XVI.] 



CoxrE>'TS. 



I. Introduction. 



II. Carbonaceous Eocks or Culm-measures. 

 III. Devonian Eocks. 



1. Beds below the Plymouth and Torbay Limestones. 



2. The Plymouth and Torbay Limestones. 



3. Beds overlying the Plymouth and Torbay Limestones. 

 JV. Metamorphic Eocks of the Salcombe District. 



V. Dartmoor and Erown Willy Granite. 

 VI. Greneral Eemarks. 



I. Introduction^. 



In the course of last year I made an examination of the older rocks of 

 South Devon and the adjacent portions of Cornwall, for the purpose 

 of ascertaining the stratigraphical relations of the different beds, or 

 groups of beds, with a view to their coordination with the more 

 complete and better-known series in the northern part of the county. 

 I was led to undertake this in the belief that, notwithstanding the 

 many memoirs that have appeared on Devonian Geology, there was 

 still a very grekt difference of opinion among geologists respecting 



* This memoii- is illustrated by an Ordnance Map coloured geologically, from 

 whicli PI. XVI. lia? been reduced. 



