402 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY, [April 22, 



Devonian rocks, they give to the two formations a general faeies more 

 similar in the southern than in the northern part of the county ; and 

 their separation is consequently more diiScult. But although much 

 of these slaty rocks bears a strong resemblance in lithological character 

 to those of the underlying series, and occasionally gives rise to much 

 perplexity in deciding to which system particular portions should be 

 i-eferred, nevertheless they differ as a whole in a manner which is 

 sufficiently obvious, but which it is difficult to convey in words. Por 

 instance, the underlying rOcks along the southern border of the Culm- 

 measures are almost entirely devoid of grits, and consist essentially 

 of fine argillaceous slates, more or less mixed with volcanic matters, 

 and often much affected by cleavage. The higher series, on the con- 

 trary, is distinguished more particularly by the thin seams of green - 

 ish-grey grit, varying from half an inch to 2 or 3 inches in thickness, 

 which they contain. These grits sometimes become locally more 

 developed, and then constitute beds of greenish or greyish sandstone 

 of considerable thickness, separated only by thin seams of slate ; 

 while in other places the thin grits are represented by thinly bed- 

 ded black, or black and white, chert interstratifiedwith dark-coloured 

 slate, both the chert and the slate having a tendency to become 

 bleached in weathering. Carbonaceous matter is not by any means 

 so generally distributed, nor so characteristic as might be expected ; 

 and cleavage, although not absent from the more purely argillaceous 

 portions of the series, is not so frequent as in the lower rocks. 



The southern limit of these Carbonaceous rocks ranges from the 

 coast at Boscastle by Lesnewth to Hallworthy, and thence along the 

 south side of Laneast Down, and the north of Trewen, - to a north 

 and south fault, which crosses the Launceston turnpike-road west of 

 Kenners House, and is laid down in the maps of the Geological Sur- 

 vey. This fault throws down the Culm-measures on the eastward, 

 and brings the line between them and the underlying slates south- 

 wards to Congdon. From Congdon the line passes by Bolathan to 

 the north side of the brook at Does Houses, where a few beds of 

 slate separate the Culm-measures from the northernmost of the 

 three Petherwin limestones. The Culm-measures dip to the north, 

 and preserve this dip, undulating at angles which vary from 5~ to 

 15°, to within half a mile of Launceston, where it becomes reversed. 

 A little further doA^-n the stream, on its north side, on the road from 

 Launceston to Landlake, there is a quarry of dark-blue or nearly 

 black slate, with thin seams of grey grit. The slates contain Posidovo- 

 mya, and dip south at from 5° to 10°. On the opposite side of the 

 brook are the fossiliferous grey slates overljing the limestone which 

 has yielded the rich but peculiar Cephalopod fauna of Landlake. Both 

 the slates and tne limestone dip jS". 30° E., at an angle of about 20° ; 

 . while the Culm -measures dip towards them mth a lower but some- 

 what undulating dip. A quarter of a mile, or rather less, to the 

 S.S.E. of the limestone, black chert is quarried in a field on high 

 ground, dipping E. 25° S., at alow angle, with much contortion ; and 

 a little further down the brook similar beds are exposed near the 

 bridge under Hardow Down, overlain by dark -grey flags with plant- 



