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and the lesser glacier of the hill of Nigg, sloping towards the north, saw themselves reflected in the separating strait of sea which at this remote period flowed through the flat valley between. The valley is still occupied for half its length by a sandy estuary, known as the Sands of Nigg, which ere the upheaval of the higher beaches, must have existed as a shallow channel, through which the Firth of Cromarty,—then a double-mouthed arm of the sea, with the hill of Nigg as a mountainous island in the midst,-communicated with the Moray Firth beyond.

PHENOMENA EXPLANATORY OF ACCUMULATIONS OF SHELLS.

THERE are scarce any of the appearances with which the geologist is conversant more mysterious than the immense accumulations of shells which he occasionally finds, as in some parts of Sweden, separated from all extraneous matter, as if they had been subjected to some sifting process, cleaned, as it were, and laid by; and it has long been a question with him how this sifting process has been effected. The theory that the accumulation had been heaped up by great floods, through which substances of the same specific gravity were huddled together, has been the commonly accepted one; but who ever saw a flood, however great, that did not cast down its mud and its clay among its transported shells, or that had not mingled them, in the process of removal, with its lighter gravels or its sand? In the flat estuary of Nigg, I have seen the sifting process effected through a simple but adequate agency. For about two miles from where the estuary opens into the Cromarty Firth, its wild tracts of yielding sand are thickly occupied by the shells that love such localities,-in especial, by the common cockle. Almost every tide, when the animals are in season, furnishes its vast quantities for the markets of the neighbouring towns, and still the supply keeps up ample as at first. Now the tracts of sand which they inhabit, if not properly quicksands, are at least extremely loose, especially when covered by the tide ; and though the creatures succeed, so long as they live, in maintaining their proper place in them within a few inches of the surface, no sooner do they die than the shells begin gradually to sink downwards through the unsolid mass, till, reaching, at the depth of about six feet, a firmer stratum, they there accumulate, and form a continuous bed. The work of accumulation has been going on for many centuries ; generation after generation has been dying, to undergo this process of burial,—this process of subarenaceous deposition, if I may so speak; and there are places in the estuary in which the shelly stratum has risen to within a foot or two of the surface. It forms a sort of quarry of shells; and when, about thirty years ago, there was a lime-work established in the neighbourhood, many thousand cart-loads were dug out and burned into lime. I had frequent occasion, some five or six years since, to pass through the estuary at seasons when the mere amateur would have perhaps stayed at home. There runs through it a stream of fresh water, that drains the flat fields and scattered lochans of Easter Ross; and on one of my winter journeys, after a sudden thaw, accompanied by heavy rains, I found the stream swollen to the size of a considerable river, and its bed excavated beneath the usual level some three or four feet, with the sectional line of sand and shells through which it had cut standing up over it like a wall. There was first, reckoning downwards, from a foot to eighteen inches of pure sand; and next, from two feet to two feet and a half of dead shells. The sandy tract all around, for many hundred acres in extent, used to be partially covered with water; every furrow of the ripples, and every depression of the surface, borrowed its full from the receding tide, and, from the general flatness, retained it till its return. But on this occasion, the surface-water had found an unwonted drainage, through the upright sectional front, into the newly excavated bed of the stream. It sank through the upper arenaceous layer as through a filtering stone, and then came rushing through the stratum of shells underneath, brown with the sand which it swept from their interstices. Nor could there be a completer sifting process. For yards and roods together the shells were as thoroughly divested of the sandy matrix in which they had lain as if they had been carefully washed in a sieve. I was bold enough to infer from the phenomenon at the time, that the problem of the unmixed accumulations of shells may be, in at least some cases, not so difficult of solution as has been hitherto supposed. One has but to take for granted conditions such as those of the estuary of Nigg,--the incoherent bed, half a quicksand, and the subarenaceous deposition,—to account for their original production, and the superadded conditions of the surface-water and the free drainage, to account for their after clearance of extraneous matter.

CAUTION TO GEOLOGISTS ON THE FINDING OF REMAINS.

In consolidated slopes it is not unusual to find remains, animal and vegetable, of no very remote antiquity. I have seen a human skull dug out of the reclining base of a clay bank, once a precipice, fully six feet from under the surface. It might have been deemed, not without a degree of plausibility, the skull of some long-lived contemporary of Enoch, -perchance that of one of the accursed race,

• Who sinned and died before the avenging flood.'

Nay, a fine theory was in the act of being formed regarding it, which affected the whole deposit ; but, alas! the labourer dug a little further, and struck his pickaxe against an old Gothic rybat, that lay deeper still. There could be no mistaking the character of the chamfered edge that still bore the marks of the tool, nor that of the square perforation for the lock-bolt; and the rising theory straightway stumbled against it and fell. Both rybat and skull had come from an ancient burying-ground, situated on a projecting angle of the table, and above.

REMARKS ON UNDERLYING CLAY ON LEVEL MOORS.

On level moors, where the rain-water stagnates in pools, and a thin layer of mossy soil produces a scanty covering of heath, we find the underlying clay streaked and spotted with patches of white. As in the spots and streaks of the Red Sandstone formations, Old and New, the colouring matter has been discharged without any accompanying change having taken place in the mechanical structure of the substance which it pervaded; for we find the same mixture of arenaceous and aluminous particles in the white as in the red portions. And the stagnant water above, acidulated, perhaps, by its various vegetable solutions, seems to have been in some way connected with these appearances. In almost every case in which a crack through the clay gives access to the oozing moisture, we find the sides bleached, for several feet downwards, to nearly the colour of pipe-clay; we find the surface, too, when divested of the soil, presenting for yards together the appearance of sheets of half-bleached linen. Now, the peculiar chemistry through which these changes are effected might be found to throw much light on similar phenomena in the older formations. There are quarries in the New Red Sandstone in which almost every mass of stone presents different shade of colour from that of its neighbouring mass, and quarries in the Old Red, whose strata we find streaked and spotted like pieces of calico. And their variegated aspect seems to have been communicated in every instance, not during deposition, nor after they had been hardened into stone, but when, like the boulder-clay, they had existed in an intermediate state.

TRAVELLED BOULDERS NOT ASSOCIATED WITH CLAY.

All the travelled boulders of the north do not seem associated with the clay: we find them occurring, in some instances, in an overlying gravel, and in some instances resting at high levels on the bare rock. I have seen, on the hill of Fyrish,-a lofty eminence of the Lower Old Red which overlooks the upper part of the Cromarty Firth,-a boulder of an exceedingly beautiful, sparkling hornblende, reposing on a stratum of yellow sandstone, fully a thousand feet over the sea, where there is not a particle of the clay in sight. We find these travellers furnishing specimens of almost all the primary rocks of the country,—its gneisses, schistose and granitic, its granites, red, white, and grey, its hornblendic and micaceous schists, and occasionally, though more rar its traps. The stone most abundant among them, and which is found occurring in the largest masses, is a well-marked granitic gneiss, in which the quartz is white, and the feldspar of a pink colour, and in which the mica, intensely black, exists in oblong accumulations, ranged along the line of stratification in interrupted layers. No rock of the same kind is to be found in situ nearer than thirty miles. We find granitic boulders of vast size abundant in the neighbourhood of Tain, especially where the coach-road passes towards the west through a piece of barren moor, and on the range of sea-beach below. One enormous block, of a form somewhat approaching the cubical, is large enough, and seems solid enough, to admit

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