304 NOTES. 



naturally induced to suggest that the beds with Cephalaspia 

 would be found to lie above the fish-beds of Cromarty and 

 Caithness. In looking, however, to the physical order of 

 the masses in that northern region (p. 280), we see that 

 this view cannot be retained ; for the bituminous schists of 

 Caithness are comparatively high in the series, and, resting 

 upon a great thickness of sandstone and conglomerate, are 

 overlaid by the upper zone of the group only. According 

 to my view, therefore, as founded also on the natural sections 

 in Shropshire and Herefordshire, the conglomerates and sand- 

 stones which underlie the flagstones of Caithness are the equi- 

 valent in time of the lower cornstone strata of England. This 

 determination is of considerable importance, since good geolo- 

 gical text-books, including the last edition of Lyell's ' Manual' 

 and Page's * Advanced Text-Book,' — ^both following Hugh 

 Miller, — have placed the Caithness flags in the lowest division 

 of the Old Red Sandstone.* We may also, indeed, clearly in- 

 fer that, even if the Arbroath paving-stone, with their Ptery- 

 goti, do not represent the uppermost Ludlow rocks, still it 

 follows that the Cephalaspis beds of Forfarshire must fall 

 into the lower division of the Old Bed group." 



"With regard to the superposition of strata in Orkney, Sir 

 Roderick says, — 



" When in company with Mr Peach, I was re-assured that 

 the same flagstones as those of Caithness, and containing 



* "In formerly adopting the belief that the comstones, with Cepha- 

 laspis, generally represented the middle beds of the Old Red Sandstone, 

 Hugh Miller was quite justified ; for it was then supposed, even by my- 

 self, that these concretions occupied the central part of the group ; whilst 

 we now know that their inferior portions actually graduate downwards 

 into the tilestones and summit of the Ludlow rock. Again, it was for- 

 merly believed that a Dipterus (a marked Caithness geniis) had been 

 found in the Upper Ludlow rock. This was a mistake in the original 

 'Silurian System ;' for in no subsequent researches has the smallest frag- 

 ment of a Dipterus been detected in the bone-bed of the Upper Ludlow 

 rot^k." 



