128 The Rev. D. Williams's Proofs that the Devonian 



Survey was necessarily so limited. That accomplished geolo- 

 gist, however, departed somewhat from his characteristic cau- 

 tion, when (pp. 195 and 196) he states that "the Petherwin 

 slates rise from under the black carbonaceous shales ;" and I 

 venture to hope, on re-perusing the section on the " Petherwin 

 Group," he will correct the only blemish I have noticed in his 

 admirable work. The evidences about Landlake are of them- 

 selves, I take it, altogether too equivocal to justify our forming 

 a judgement one way or the other; my original inference that 

 the Trilobite slates of Exmoor, No. 7, were brought up in 

 the south, in the same order of succession in which they de- 

 scended on the north, was however deduced from the same 

 premises and localities (according to his description) as that 

 drawn by Mr. Phillips, viz. a traverse I made from Launces- 

 ton due south by " Bad Ash " to the Clymenien limestones 

 at Landlake. 1 was betrayed into error by the tempting order 

 of succession, by a supposed higher antiquity of the killas, and 

 the doubtful nature of the evidences: the following is a brief 

 detail of what I since observed. 



The barren ridge of St. Stephen's Down, a mile north of 

 Launceston, is the Coddon Hill grit. No. 8, and a portion of 

 the great south anticlinal axis before noticed. South of this the 

 overlying culm grits and slates are carried to near the Land- 

 lake lime quarries by an easy undulation, as from Tresmar- 

 row to Petherwin, before noticed. In a large abandoned quarry 

 immediately north of the brook, the carbonaceous slates dip 

 south 10°, and in an old low cutting in the road below, have 

 an apparent tendency to flatness. Twenty paces further con- 

 duct us to the little brook, and fifty or sixty more to the killas 

 and Landlake limestones ; if, however, having crossed the 

 brook, we turn into a gate behind the cottage, and follow an 

 old road-way through the wood, till we attain the first open 

 field, we shall see a small stone quarry for the roads : this is 

 undoubted Coddon grit; it is situated in the rising ground a 

 little south of the brook and valley ; dips at a low angle some- 

 what north of N.E., and must be very near the northern foot 

 of the Landlake limestone and killas*. 



Having returned to the road by the cottage, and ascended 

 a short distance south, we observe a spoil heap of the killas 

 and lime rocks close on our right hand, and immediately on 

 our left, over the fence, near the old lime-kiln, a deep old 

 slate quarry ; but this is a very different slate from that on our 

 right-hand above ; il underlies it, and is almost destitute of 



* It is right to observe that isolated clips like this, in the Coddon Hill 

 grit, are never to be relied on, its beds, in this respect, in their descent 

 from the plane to tiie base of a hill being oftentimes full of contradictions. 



