OLD RED SANDSTONE OF WESTERN EUROPE, 413 
contrast singularly with a white and reddish enamelled ichthyolite, often lying 
on its back or jumbled asunder in the heart of a nodule from Lethen Bar, or 
with a grey bone-coloured specimen from Gamrie, showing, perhaps, only a few 
plates or bones, but all in an admirable state of keeping. It is only after a 
great many specimens exhibiting all these varieties of preservation has been 
examined that a paleontologist can secure himself against the risk of multi- 
plying species. I quite anticipate, that as the able naturalists who named the 
ichthyolites from the north of Scotland had not the advantage of this extended 
experience, there will be considerable pruning of the fossil lists when they are 
revised by a competent authority. 
It will be seen from Table II. (p. 452) that the great preponderance of the 
Orkney ichthyolites belong to genera, and in many cases even to species, charac- 
teristic of the higher groups of Caithness, and found also along the western 
and southern shores of the Moray Firth. The acanthodean fishes are specially — 
noticeable, since they have not been observed among the Wick flagstones. 
Cheiracanthus and Diplacanthus are well represented. Cheirolepis, a Moray 
Firth form, likewise occurs. The wide-ranged Coccosteus is common in Orkney, 
likewise Asterolepis Asmustt, Diplopterus (three species), Osteolepis (the two 
abundant Caithness forms and 0. brevis of M‘Coy), and the long-enduring 
species of Dipterus (D. macrolepidotus). ~The forms of Glyptolepis so common 
on the south side of the Moray Firth, as well as in the higher Caithness groups, 
including also the so-called Holoptychius Sedgwickii, are found in Orkney. A 
further point of connection between the flagstones of these islands and the fish- 
bearing strata of the Moray Firth, is furnished by the occurrence in both 
| districts of two species of Pterichthys (P. cancriformis and P. Miller). 
4. Origin of some Features of Orkney Scenery.—In quitting Orkney I 
| would refer to the many admirable sections exposed along the mural sea- 
cliffs of that storm-swept group of islands, and to the endless instructive 
lessons furnished by them on stratification, jointing, and other elementary 
questions in physical geology, as well as on the progress of weathering, 
its relation to rock-structure, and the proportional share taken in it by the 
| sea and the atmosphere. As a rule the flagstones crumble slowly; they 
| form, indeed, a most durable material, and it is well for the Orcadians that 
| their cliffs stand up so stoutly against the dash of rain and sea-spray. But like 
those of Caithness, they are traversed by many joints, and it is through these 
diagonal lines that they are mainly broken up. Frost is there a comparatively 
feeble agent. Nevertheless, with the co-operation of wind, rain, and waves, the 
lines of joint are slowly opened into narrow vertical rifts, extending from 
| bottom to top of a sea-wall 150 or 200 feet high. These might not unnaturally 
k regarded as rents due to earthquake shocks. By the widening of these 
| VOL, XXVIII, PART IL. 5P 

