Vol. 59.] THE TINTAGEL AND DAVIDSTOW DISTRICT. 411 



(b) Maceoscopical Description oe the Rocks. 



The Tredorn Beds. 



The uppermost beds of the series of rocks described in the 

 present paper have been traced from Abbott's Hendra near St. 

 Clether to the coast, a distance of about 8 miles. Taken as a 

 whole, they are light bluish or greenish-grey phyllites, with 

 slightly marked foliation-surfaces ; usually soft enough to be 

 scratched by the nail, and often speckled by elliptical rust-red spots. 

 Occasionally they are dull greyish-black, either splitting readily 

 into thin sheets, or, locally, into less well-defined slabs. 



From the London & South-Western Railway to the coast, small 

 white spots (probably of orthoclase) are characteristic of these slates ; 

 east of the railway they have not, so far, been found (Abbott's Hendra 

 type, identical with the slate of Caroline Quarry on the coast). 



In the western part of the district these rocks near their base, 

 that is, near to the Upper Blue-Black Slates, recall the underlying 

 Hallwell-Cottage Beds ; while the latter in their uppermost part, 

 that is, near to the Lower Blue-Black Slates, are often white- 

 spotted, and recall the Tredorn Beds. At Caroline Quarry, north 

 of Trebarwith Strand, the rock is a dull, greenish-grey, fossiliferous 

 slate, with a rather ropy fracture, whereas close by at Lanterdan 

 Quarry it is darker, more slaty, and, on the whole, harder ; 

 a common type, recalling many other rocks from very different 

 horizons. 



The Volcanic Series. 



These are blue-greyish, or, more commonly, greenish rocks, vary- 

 ing much in appearance from point to point, but possessing characters 

 which, as a whole, clearly separate them from beds above and below. 

 Schistose structure is generally very marked; foliation-surfaces glitter 

 with brown or bronze-coloured mica-scales, which often are rather 

 patchily distributed, and not infrequently are associated with 

 crystals of epidote. 



A banded or lenticular structure is locally very conspicuous, 

 occasioned by the presence of carbonates ; and crystals of felspar 

 catch the eye in a few specimens. 



The result of decomposition in these rocks is to produce a 

 characteristic, very fine, bluish mud usually containing mica, 

 which is of great help in mapping where exposures are scarce. 

 With rocks of this type are associated, on the one hand, many more 

 slaty in character ; on the other, specimens strongly recalling the 

 German schalsteins. 



This somewhat heterogeneous group is termed the * Volcanic 

 Series/ and has been found to afford a well-marked datum-line, 

 which, in the great majority of instances, may be readily recognized 

 from fragmentary exposures. To this rule, however, may be found 

 local exceptions, as, for example, in the neighbourhood of Trambley 

 Cove. There, these rocks are hard and slaty, no doubt of the nature 

 of passage-beds below the Upper Blue-Black Slates, but differing 

 considerably from the typical members of the Volcanic Group. At 



