OOLITIC SYSTEM OF SCOTLAND.

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shallow, and rather thickly set. The circular elevation, whichcorresponds in this genus to the inclosing wall of Isastrea, isvery apparent in weathered specimens, but, as in Thamnas-trea Lyelli, only faintly visible in those that are less worn;while, as in the species Thamnastrea scita, the columella, ifit at all possessed one, was rudimentary. I have usuallyfound this species encrusting masses of indurated Old RedSandstone of the flagstone formation, which must have beenas ancient a looking rock in the times of the Oolite as it isnow, and, when laid open by the waves along the beach, musthave exhibited its ichthyolitic remains in their present stateof keeping. In fine, in its rocks and stones this beach of theOolite on what is now the eastern coast of Sutherland musthave resembled that of the neighbouring county of Caithnessin the present day. And as on the latter shore, as we ap-proach the line of extreme ebb, we find rolled masses of darkgray flagstone, partially covered with pale-coloured nulliporiteencrustations, there would have been found, had there beenan inquiring eye to prosecute the search, similar dark graymasses, bearing their encrustations of Thamnastrea, along theold shores of the Oolite.

But while the framework of the scenery must have beenthus the same in both eras, and the same incalculably ancientsea must have broken in both against the same old fossil-bear-ing rocks, how entirely different must not the vital sceneryof the two periods have been ! Where we now see micro-scopic Lepralia and dwarfish Sertularia, huge Isastrea, embroi-dered by their flower-like polypes, and wide-spreading sheetsof Thamnastrea, similarly mottled, must have gleamed whitethrough the green depths of the water, as their existing re-presentatives may be seen gleaming from the quiet recessesof tropical lagoons in the present day • the ammonite and be-lemnite must have careered over and around them amid thasheen of ganoidal scales ■ and where the seal now disports,