534 C. Lapivorth — Researches in the GraptoUtic Shales. 



held by diflferent geologists concerning tlie tbickness and geological 

 age of the carbonaceous Graptolite-bearing sbales of South Scotland. 

 He expresses his conviction, derived from a study of the fossils they 

 afford, that there is but " a single band of Graptolite shale, which 

 runs through the Llandeilo beds of South Scotland, there being in 

 this band several distinct zones, each marked by a different assem- 

 blage of fossils, but with many species in common." ^ 



Having devoted considerable attention to these peculiar strata 

 during the last few years, I wish, in anticipation of a detailed paper 

 on the whole subject, to give a very brief summary of the main 

 conclusions at which I have arrived, more especially in reference to 

 those points to which Mr. Hopkinson has alluded. 



Professor Harkness, who first called the attention of geologists 

 to these beds, gave it as his opinion that the relations of these Black 

 Shale bands to each other is such as to prove that they were 

 originally portions of the same deposit; and that their present 

 position is probably attributable to a succession of faults, which run 

 through the district in the direction of the strike of the rocks, and 

 have repeatedly thrown down the strata in a N.N.W. direction.^ He 

 afterwards somewhat modified this opinion, at the suggestion of Sir 

 E. Murchison, and explained their present relations by great curva- 

 tures of the strata, the upper portions of which have been denuded.^ 

 He also believed that the rocks in which these bands are imbedded 

 are the equivalents of the Caradoc Sandstone ; while Sir E. 

 Murchison considered that they more nearly represent that great 

 mass of Welsh Schist which underlies the Bala Limestone, i.e. of 

 Upper Llandeilo age.* 



The latter view of their age has been unhesitatingly adopted by 

 the great majority of Silurian geologists, and it will require no 

 slight weight of contrary evidence to displace it. 



The theory of Professor Harkness, that these bands are simply 

 repetitions of one and the same deposit, has gradually given way to 

 another, viz. that there are several distinct bands, or groups of bands, 

 on very different geological horizons, and divided from each other 

 by great vertical thicknesses of comparatively imfossiliferous strata. 

 This latter theory has been lately adopted by the Geological Survey 

 of Scotland. In the carefully prepared " Explanation to accompany 

 Sheet 15," the Silurian Bochs of Lanark and the North of Dumfries 

 are arranged in descending order, as follows : — ■ 



H. Caradoc or Bala Beds? 

 Llandeilo Beds. 



(g.) Blach Shale Group. 3400 feet. 



(f.) Lowtlier Group. 5000 feet. 



(e.) Haggis Boch Group. 1800 feet. 



(d.) Balveen Group. 2900 feet. 



(o.) Daer Group. Thickness not ascertained. 



(b.) Hartfell [Black) Shale Group. „ 



(a.) Queensberry Grit Group. „ 



■^ Page 501. * Geol. Journ., toI. vii., pp. 51, 52. 



3 Geol Journ. , vii., p. 162 ; xii., p. 245. * Geol. Journ., vii., pp. 53, 162. 



* Explanation, Sheet 15, Geol. Surv. Scotland, p. 9. Edra., 1871. 



