1823.] Geology of Devon and Cornwall. 185 



distinguished either by the magnificence of their scenery, or the 

 extent of the sections and singularity of the phenomena which 

 they exhibit. Many and valuable as the contributions to the 

 geological history of both counties have unquestionably been, 

 yet our knowledge of their structure (especially in the case of 

 such districts as do not fall under the above-mentioned heads) 

 is by no means so perfect as to preclude even in a casual visitor 

 the hope of adding somewhat of information to the present 

 stock. It may be useful too to point out the deficiencies which 

 yet remain to be supplied by those who have better opportunities 

 and leisure. Such is the object of the following memoranda 

 collected chiefly during the summers of 1809 and 1812, and 

 partly verified in that of 1819. On many points they are of 

 necessity very imperfect, and some parts of the original manus- 

 cript have been omitted in consequence of the same phenomena 

 having been far more accurately and fully described in the essay 

 contributed by Mr. Sedgwick to the first part of the Cambridge 

 Philosophical Transactions. What is retained, I have arranged 

 for convenience' sake geologically, rather than topographically ; 

 and in this arrangement I venture to propose the following divi- 

 sion of the principal rock masses as one which, if not strictly 

 scientific, will yet, I think, be found useful for the purpose. 



1. Granite, including some porphyritic beds and mineral 

 veins, and shorl rock. 



2. Metalliferous, or, more strictly, cupriferous and stanniferous 

 slate, including various porphyritic and felspathic rocks (elvans), 

 and occasionally greenstone. This I will venture for brevity's 

 sake to. term the inferior slate. 



3. Slate (which I shall venture for the same reason to term 

 superior), containing no elvans, but abounding much more in 

 greenstone, especially in its obscurer varieties, and in dark 

 coloured limestones. Sparingly metalliferous, containing no tin, 

 and more productive of lead than the inferior. Contains occa- 

 sionally organic remains. 



4. Stratified rock, exhibiting the general character of a con- 

 glomerate or sandstone, alternating with tender slate, and occa- 

 sionally associated with coralline or shelly limestone. Contains 

 no metallic veins, and few if any rocks of the greenstone 

 species. This rock might, perhaps, be regarded as forming the 

 upper portion of No. 3, and both would probably by most geolo- 

 gists be termed greywacke. As any attempt at restricting that 

 term might produce confusion, I shall venture for the present to 

 term this (No. 4) sand slate.* 



* It is, perhaps, almost needless to remind those who are practised in geological research, 

 that the division of slates here adopted is purely arbitrary, and, for convenience, as the 

 inferior do in fact pass by so imperceptible a gradation into the superior as to render it 

 impossible to assign any determinate limits to each variety, however an examination of 

 points distant inter »e may satisfy us of their characteristic difference. It appears to be 

 pretty generally agreed that such a gradation is observable in most of our schistose 

 range?. 



