•414 A. Cl/ampcnioicne — Age of the Ashburfon Limestone. 



line, and for a short distance from the contact, the colour is gone 

 from the slates. Instances of this can be seen conspicuously in a 

 quarry at the Northern foot of Knowlehill near Newton Abbott, and 

 again in the cutting near Combe cellars, where one or two small 

 tongues a few feet thick appear isolated in the discoloured slates 

 which surround them. They are probably only branches from the 

 great mass that runs up to Eowdon Cross. 



Thus we see that these coloured slates which run S.E. of the 

 Ashburton limestone pass North of the Ogwell mass, and South of 

 that of Bishopsteign ton, throwing them off on opposite sides. They 

 form the crest of a denuded and overturned anticlinal line, and 

 clearly correspond with the coloured slates North of Plymouth, at 

 Mutley, etc., to which district we must now turn our attention. 



The apparent thickness of slaty rocks North of Plymouth is 

 enormous. An excellent paper" On the Geology of Plymouth " was 

 written by Mr. Worth, F.Gr.S., for the Trans. Plymouth Institution, 

 1875. In that paper (p. 9) he pointed out that three or four sub- 

 parallel belts of coloured slates, separated by slates of the ordinary 

 type, are doubtless only one group, repeated by folds, the same 

 general dips to the south at high angles prevailing throughout, 

 making the apparent thickness vastly greater than the actual. 

 Having formerly stayed some months at Knackersknowle, I can 

 testify to the truth of these statements. The folds, however, are 

 certainly more numerous than in the corresponding South-East 

 Devon area, and the purple slates, their axes less thrown over, 

 rise in greater bulk. 



Dr. Holl has shown how, through a series of folds, the Liskeard 

 and St. Cleer igneous rocks are brought down to Saltash,^ and 

 Mr. Worth has pointed out bow close the igneous bands come down 

 to the outcrop of the limestone at Devonport f so that one may now 

 confidently predict that the so-called " Liskeard and Ashburton 

 group " of Sedgwick and Murchison will resolve itself into a 

 complicated series of folds of the " Plymouth and Torquay group," 

 limestone becoming scarce North-west from Plymouth, or possibly 

 not being " brought in " among the folds. But at any rate such 

 fossils as occur in this direction, especially at Liskeard, have been 

 shown by Dr. Holl to be of a Middle Devonian type ; ^ there is, in 

 fact, no sign of a Lower Devonian or Coblentzian fauna or rock 

 group in the neighbourhood of Liskeard, whatever the beds of the 

 Looe river section, or those of Whitesand Bay, may be. 



In contrast to this let us compare, first, a small tract comprising 

 strata known to be of Lower Devonian age,* the Torquay Promontory, 

 with, secondly, the slate rocks expanded West of the great lime- 

 stones of Ipplepen, etc. In the former (saving some calcareous 

 shales with lower Eifel Orthides, Phacops latifrons, Spirifera speciosa, 

 etc., which peep out from under greatly faulted limestone in parallel 

 stratification), we have a series of tough grey and brownish grau- 

 wacke grits and schistose beds with fossils of the Coblentzian series, 



1 I.e. p. 442. " I.e. p. 7. ^ l-c. table iii. p. 450. 



* Murchison, Siluria, p. 398 ; Salter, Q.J.G.S. 1863, p. 4S3. 



