C. Lapworth — Researches in the Graptolitic Shales. 535 



Two distinct groups of Black Shales are here recognized, and they 

 are supposed to be divided from each other by at least 10,000 feet 

 of intervening unfossiliferous strata. The whole series is referred to 

 the Llandeilo generally, and the higher Black Shale Group is classed 

 unhesitatingly with the Upper Llandeilo. 



In a paper read at the meeting of the British Association in 

 Edinburgh, in August, 1871, and which was afterwards published 

 in the Geological Magazine,^ Mr. Jas. Wilson and myself ex- 

 pressed our belief that there was but a single band of Black Shale, 

 that it lay at the summit of the Moffat Series, and probably formed 

 a connecting link between the Llandeilo and Caradoc formations. 

 A short notice of the general results of my later investigations was 

 given in a paper read before the Geological Society of Glasgow in 

 January last, an abstract of which appears in the present part of the 

 Transactions of that Society. 



Subsequent researches have simply more clearly proved the cor- 

 rectness of my main conclusions as there given, which may be thus 

 very briefly summarized : — 



There is but a single group of these carbonaceous and graptolitic 

 shales, of from 500 to 600 feet in total thickness. This group, 

 which may for the present be called the Moffat Shale, is litho- 

 logically and palseontologically separable into three great divisions, 

 viz. the Loiver, Middle, and Zfpper Moffat. These divisions, which 

 are very different in vertical thickness, each contain a great abun- 

 dance of Graptolites; and though there are probably nearly a 

 hundred different species in all (of which at least one third are as 

 yet undescribed), yet so restricted are they in their vertical distribu- 

 tion, that it is impossible to say at present that there are half a dozen 

 forms which are not peculiar to one or other of these divisions. 



The major divisions naturally subdivide into several distinct zones, 

 each characterized either by the exclusive possession of some well- 

 marked species, or by the constant presence of some peculiar group 

 of species. These zones are easily recognizable, and furnish exactly 

 the same fossils, in localities as much as forty or fifty miles apart. 



The Lower Moffat contains the Graptolites of the Hudson Biver 

 Group of America, and those of the Llandeilo beds of Portmadoc, 

 and is, in my opinion, of Lower Llandeilo age. The Middle Moffat 

 appears to be the equivalent of the Upper Llandeilo of Builth; 

 while the Upper Moffat is decidedly Caradoc. The Moffat Shale, 

 which in almost every case comes to the surface along anticlines 

 of the strata, passes up conformably into the overlying Gala Group, 

 (which belongs to the Up-per Caradoc and Llandovery period) in 

 several localities in the southern districts, but to the north the 

 basement bed of the latter rests usually upon the Loioer Moffat beds, 

 so that there appears to be in many cases an entire absence of Upper 

 Llandeilo and Lower Caradoc rocks in that direction, the Lower 

 Llandeilo being succeeded immediately by the Upper Caradoc. 



1 Vol. VIII., p. 463. 



