great graywacke system of Somerset, Devon, and Cornwall. 73 



the road to Launceston, similarly associated, with the additional ac- 

 companiment of intercalated killas and Coddon Hill grit dipping 

 beneath the volcanic ash of Marly Mead. Mr. Williams also mentions 

 a shaft sunk in a copper lode close by the turnpike gate on the road 

 from Tavistock to Callington, as another instance of floriferous grit, 

 or a rock exactly the same as that which contains the copper lode of 

 Wheal Friendship, underlying pale green slate or killas : and he 

 adds, there can be no doubt that Wheal Friendship mine is in No. 9 

 or the floriferous series. The next instance mentioned in the paper, is 

 at Linkinghorn and South Hill, north of Callington, where there is 

 stated to be an entire suite of alternations of killas, Coddon grits, and 

 floriferous grits with plants, all dipping south. Again, near Pillaton, 

 between Callington and Saltash, there are said to be countless minor 

 alternations of floriferous and Coddon grits, each thinning out and 

 interlocking like the teeth of a trap. Another instance of the killas 

 overlying the floriferous grit, is said to occur in a hill at Penter's 

 Cross on the road from Callington to Saltash, where a cutting ex- 

 hibits in the lower part, a series of alternations of floriferous sand- 

 stone, and culmy slates which in ascending disappear, and in their 

 place a delicate pale green killas alternates with the sandstone 

 beds, while at the summit the sandstones disappear and are replaced 

 altogether by killas. A fault traverses the hill, and to the south of 

 it only killas occurs. 



The general results of his observations, Mr. Williams says, 

 show, that in the ascending order, from Cannington Park and the 

 Quantocks in Somersetshire, to the Land's End, there is a group 

 constituted of ten strikingly simple, consecutive series, severally 

 varying in their mineral and zoological character ; that as respects 

 the limestone suites, the thin and spare dimensions of such as occur 

 in the Trilobite slates of Exmoor (No. 7) render them too insignifi- 

 cant to be noticed ; that the Posidonia limestones, which by mineral 

 gradation succeed and conformably overlie the latter, are elliptically 

 included in the Coddon Hill grits (No. 8), and together distinctly 

 underlie and constitute the base of the great floriferous series (No. 9) : 

 that higher up in No. 9 is an extended horizon, which separates the 

 series into an upper and a lower, containing the Bampton, Hock- 

 worthy, Holcomb Rogus, and Hastleigh limestones, with red and 

 black slates on the north; the Petherwen and Landlake slates and 

 limestones on the south ; and the entire suite of coral limestones to 

 the east of Dartmoor, extending from Chudleigh to Berry Head 

 and Brixham : that the Plymouth limestones included in the killas 

 (No. 10) are higher in the group than those just mentioned, and 

 are introduced first at Millaton, about a mile and a half west of St. 

 Germains. 



Mr. Williams considers the slate or killas series of S. Devon to be 

 distinguished from the slates of Exmoor by a peculiar extraneous 

 cleavage. In Exmoor the cleavage, he says, is at all angles, from 

 less than 10 to the vertical, its planes having a direction of about 

 east and west ; whilst the cleavage lines of the killas either coincide 

 with the magnetic or true meridian, or depart from it to the east or 



