Introduction

In the preceding memoir the whole succession of the inferior crystalline and stratified rocks having been indicated, the triple arrangement of the Old Red Sandstone in an ascending order was shown to consist of a lower red sandstone and conglomerate, of a central deposit—the grey Caithness Flags, and of certain overlying sandstones, occasionally red, but of prevailing yellowish colour.

In this manner the whole of the Old Red series (or the equivalent of the Devonian rocks of other countries) is exhibited in the Orkney Islands, Caithness, and Easter Ross.

The lower division of the series in those tracts has not (see above, p. 400) afforded any of those fossils (the Pterygotus, Cephalaspis, Pteraspis, or Parka decipiens) which characterize the lowest Old Red of Forfarshire, Perthshire, Shropshire, and Herefordshire: the middle division (or Caithness Flags) is abundantly characterized by ichthylites and the small crustacean Estheria: the third, as known to the north of the Moray Firth, has afforded, as yet, certain terrestrial plants only, which, approaching to the Carboniferous types, are forms hitherto unknown in any true Carboniferous deposit.

In following this natural physical group westwards along the north coast from Caithness into Sutherland, or northwards from the Orkneys into the Shetland Isles, its bituminous flagstone or central portion is seen to thin out. Such is also particularly the case in the southern extension of the group; which we proceed to consider.

Thus, after passing along the east coast of Sutherland, where the lower member only is visible, it is already