1846.] DICKINSON ON JACKSTONES. 131 



Cardiola interrupta, Orthoceratites (including Creseis), and a few 

 Trilobites. The species are all Upper Silurian, — using that term in 

 the sense given to it by Sir R. Murchison. The mineral structure 

 of this formation is almost identical with that of the Upper Silurian 

 flagstones of the Lower Severn, described by Sir R. Murchison, and 

 still more exactly identical with the lower Denbigh flagstones de- 

 scribed in my paper on North Wales. 



1. Coniston limestone and calcareous slate. On an average not 

 more than 200 or 300 feet thick ; at a maximum (in Millam) about 

 600 feet thick. The fossils of this group are Lower Silurian, and I 

 need not repeat the well-known list of species. 



The whole series is overlaid unconformably by the old red sand- 

 stone and mountain limestone. 



Note. — It is right to state that in this paper the localities of Wenlock fossils 

 and those of Llandeilo flags, &c., are taken from Sir R. Murchison's descriptions 

 (excluding Marloes Bay). One or two species were added from Builth by my- 

 self.— J. W. S. 



2. On the Strata called "Jackstones" at Merthyr Tydvil. 

 By Jos. Dickinson, Esq., F.G.S. 



Considering these stones in connexion with the ironstones and 

 other strata with which they are associated, they are chiefly remark- 

 able for the peculiarity of their structure and the great quantity of 

 carbonate of lime which they contain. Few of the ironstones and 

 scarcely any of the other strata in the coal-measures in this locality 

 contain five per cent, of carbonate of lime, a\ hilst these Jackstones 

 contain forty-five. 



Their usual structure is conical, the stones being made up of a 

 series of distinct cones with serrated edges inserted into each other, 

 having the apex of each cone directed towards the top of the stra- 

 tum, although this is not invariably the case. The height of the 

 cones is various, but those most perfectly form(;d seldom exceed 

 four inches. Towards the upper and lower surface of the stratum, 

 the conical structure frequently disappears and an even arrange- 

 ment ensues. 



These stones are disposed in seams similar to those of clay iron- 

 stone. They are of a brown and sometimes of a grey colour. They 

 generally lie underneath and in contact with ironstone, and occa- 

 sionally they intervene as a band in the middle of a seam, in which 

 case, as well as in the former, there is generally a distinct line of di- 

 vision at the junction of the two stones. In many examples, how- 

 ever, the two stones blend imperceptibly with each other. 



The thickness of a stratum of Jackstones seldom exceeds nine 

 inches, and probably not more than three or four different strata 

 could be enumerated in one section. They are generally situated 

 amongst the ironstones lying below the workable coal-seams, the 

 lowest of which is about 110 yards above the mountain limestone. 



K 2 



