﻿58 J. R. Dahyns — A Question for Silurian Geologists. 



two, as well as the smooth extremity of the shields, have been broken 

 oft. The impression of the external surface, seen on the matrix, 

 shows that the axis of the thoracic segments was marked with 

 scattered tubercles. The cephalic shield and pygidium appear to 

 have been minutely granulate. On a portion of the latter these 

 granulations have a scaly form, as in some other species of the genus. 

 The ends of the pleura are broad and rounded. (See Fig. 2 a.) 



The horizon of the fossil is the ujDper part of the Arisaig series of 

 Nova Scotia, equivalent to the Lower Helderberg of New York and 

 Pennsylvania, and to the Ludlow of England. The matrix of the 

 specimen contained CJionetes Nova-Scotica, Hall ; Ilegamhonia can- 

 cellata, Hall ; Avicula Honeymani, Hall ; BeyricMa pustulosa, Hall ; 

 and other fossils characteristic of that horizon. 



Tliis fine species is evidently a close ally of H. Knightii of the 

 English Ludlow, though it differs considerably in details of structure. 

 According to Mr. Salter's determination of some specimens from 

 Arisaig placed in his hands by Dr. Honeyman, H. Knightii is also 

 found at that place ; ^ but I have not met with any specimens of it. 

 The representative of this species in the Lower Helderberg of New 

 York is II. Vanuxemii of Hall.^ 



Both of these species serve to illustrate that distinctness of the 

 Atlantic border area of North America from the inland plateau of 

 the continent, on which I have elsewhere remarked.^ Both in the 

 Upper Silurian and Carboniferous, the rocks and fossils of Nova 

 Scotia can be more easily correlated with those of Great Britain 

 than with those of New York and Pennsylvania. So much did this 

 fact strike the late Mr. Salter that he even thought it possible 

 to correlate the fossils of the Arisaig section with those of indi- 

 vidual members of the English Upper Silurian.* 



IV. — Is THERE A Base to the Cakbonifekous Eocks in Teesdale ? 



A Question foe Silurian Geologists. 



By J. E. Dakyns, Esq. ; 



Of H. M. Geological Survey of England and Wales. 



DURING a recent excursion into Teesdale I paid a visit to certain 

 Mica trap-dykes discovered by my friend Mr. W. Gunn, and to 

 a section of the Carboniferous beds below the Whin Sill of Falcon 

 Clints, near Caldron Snout, which suggested the question whether 

 there is not a base to the Carboniferous beds in that part of Teesdale. 

 The section below the Whin Sill has, I believe, been described by 

 Sedgwick. Suffice it to say that the lowest bed there exposed at 

 the very foot of the crags, just above the alluvium of the Tees, is a 

 breccia that reminds one, who knows the beds, of the breccia so 

 often found at the base of the Carboniferous in Yorkshire. The 

 next section seen in going down the stream is at the old Peucil Mill, 

 on the banks of the Tees below Cronkley Scar. Here the beds, 

 which were once wrought for slate pencils, consist of hardened 



1 Honeyman, Geol. Joui-nal, vol. xx. ^ Palaeontology of New York, vol. ill. 



^ Acadian Geology and Story of the Earth. * Honeyman, I. c. 



