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IV.—A Revindication of the Llanberis Unconformity
Published online by Cambridge University Press: 01 May 2009
Extract
This is undoubtedly the most difficult district to deal with, and one in which I have had to change my views on certain details. Still, there is one part of it where the evidence is very clear, namely, the ground between the railway and the road at the Tan-y-pant inlet.
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page 215 note 1 Professor Bonney and Miss Raisin also claim a fault on the other side of the conglomerate, between it and the main mass of the felsite, hut the junction may he seen in a block near the water's edge; one is welded to the other.
page 215 note 2 So gradual is the passage from one rock to the other that Sir A. Geikie considers the parts of the slate nearest to the felsite to be only a cleaved portion of the latter.
page 215 note 3 A similar argument is applicable to the tramway section on the opposite side of the lake. The felsite there shown next the first conglomerate is followed beyond the conglomerate (and a dyke) by nearly vertical slaty beds, but the summit of the felsite crag shows a covering of conglomerate, indicated by the presence of quartz pebbles. Here, however, the vertical succession is broken.
page 216 note 1 The following quotation from Professor Bonney's paper (C) will show that he at that time gave quite a different account of this section from mine and Miss Raisin's, and considered the conglomerates now distinguished as inseparable except as varieties. “ The cutting … passes into fine green grits or ‘bastard slate,’ beyond which we find a thick mass of interbedded conglomerate and similar jrit, then another band of grit, followed by a band of small rolled fragments of felsite about as large as hemp-seed.” It is plain that either what he here calls fine green grit or bastard slate is the same as that now called purple argillite, or he has missed the first conglomerate in the cutting, and takes the next band to be the “ Cambrian ” conglomerate, calling it “ the finer variety,” and saying that “ with considerable variety of detail the general character of these is similar.”
page 217 note 1 The manner of this was illustrated by a model at the reading of my paper.
page 221 note 1 Professor Hughes and Dr. Hicks accepted the unconformity, but I do not know that they have personally observed it.
page 223 note 1 They do not explain how they get the synclinal down again into the further valley consistently with their mapping of the heds; nor do they account for the enormous expansion of the strata between the conglomerate and slate compared with the tramway. Y Bigl summit is also represented as on slates, and not on laminated grits, but this may be an unintentional error.
page 223 note 2 It is also in accordance with Sir A. Ramsay's section.