SPLIT OR FRACTURED BOULDERS.

EVERY geologist is more or less familiar with bouldersthose rounded, smoothed, and often striated blocks so characteristic of the "Glacial Drift" or "Boulder-Clay" formation. These blocks, whether occurring in clays or in pebbly drifts, are of all sizes, from one to one hundred tons; of all shapes, from angular to rounded and polished masses; and of all the older rocks-granites, porphyries, greenstones, limestones, and sandstones-sufficiently hard to have withstood the long-continued tear and wear of ice-action. But though boulders in their ordinary aspects are familiar objects, split or fractured boulders are by no means common, and occasionally present appearances extremely difficult of explanation. We say fractured boulders, for the term split may convey the idea of some readily fissile or laminated block, whereas the phenomenon to which we refer is a fracture right across the hardest and most homogeneous boulders of granite, greenstone, and siliceous sandstone.

If we remember rightly, it was Mr Smith of Jordanhill and Mr Charles M'Laren who first drew attention to these "split boulders," assigning as the cause their falling from cliffs on which they had been perched when these cliffs had been subsequently wasted and undermined by meteoric and

aqueous action. Such an explanation may do for a boulder like that noticed by Mr Smith on the Little Cumbrae, or for that long pointed to by Mr M'Laren at the foot of Salisbury Crags, but now removed and broken up for road-metal; but it will not account for those that we find imbedded in clay or in sand, or, it may be, lying on the surface, and in all cases far removed from cliffs and rocky inequalities. We have seen several of very large size near Ballingry, in Fifeshire, lying on the surface and broken right through, the separated portions being quite adjacent, and in some instances not a couple of inches apart. Some years ago we witnessed one in a railway-cutting through the boulder - clay near Granton, on the Firth of Forth, merely cracked, and resting on another of much larger dimensions beneath it. And during the present year our attention was attracted to one at Viewforth, in the suburbs of Edinburgh, more remarkable than any other we have hitherto seen, inasmuch as it lay among the finest stratified sand, and was fractured right across the smaller portion detached only a few inches from its original place, and the interstice filled with the sand by which the whole was surrounded.

Falling from cliffs will not account for the fractures in any of the above instances, for they all lay in flat tracts far removed from cliffs; and if it should be argued that the fractures took place before the blocks were drifted from their native precipices, the argument is equally untenable, for the rounding and smoothing must have taken place subsequent to their rupture from their original sites, and even if fractured after being rounded, the chances are a thousand to one against the two portions being taken up, carried for long distances, and then dropped in such close proximity by any form of ice with which we are acquainted. In the case of the Ballingry boulders, which are of compact

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greenstone, and in many cases subangular and only partially rounded, the fracture is likely to have been occasioned by pressure, the blocks being held in the glacier or iceberg, and cracking against the subjacent rock by the unequal pressure of the stupendous mass in which they were entangled. In the case of the Granton boulder, which was of compact sandstone and of an oblong form, the fracture, which was clean and sharp, may have been produced either by pressure or by impact against the greenstone block on which it rested. But in the case of the Viewforth boulder there was no hard object either for pressure or for impact, and we can only account for its fracture by previous impact in the iceberg by which it was floated and dropped among the sandy debris. Being of an oblong shape-8 feet by 6 feet, and only between 2 and 3 feet in thickness-its fracture by the impinging of the ice - mass against some subaqueous ledge would be no difficult matter, and in this fractured state it might be held and floated about till its ultimate stranding on the spot where the two portions lay in the closest proximity. The boulder was of the syenitic greenstone of the Corstorphine Hills, beautifully smoothed and striated, and must have been fractured subsequent to this external dressing; and the only way in which its fracture and the close proximity of its parts can be accounted for is, by supposing that it was broken while held in the floating ice-mass, either by pressure or by impact against some hard and resisting object, and then left undisturbed among the sandy debris as the stranded iceberg melted

away.

The huge size and weight of many boulders, their polished and striated surfaces, their far removal from their parent rocks, and the curious positions in which many of them are perched, have long been matter of wonder and speculation; but the fracturing of masses several feet thick, and of the

hardest and toughest material, as many of these boulders are, is a subject equally deserving the attention of geologists. The explanation of Messrs Smith and M'Laren may do for the recent fracture of a few boulders that may have fallen from high and wasting cliffs, but it is wholly inapplicable in cases where no precipices are near, and, above all, for those boulders imbedded in the clays and sands of the glacial period. The mere splitting of a boulder may be accounted for by disintegration of the laminated rock, or by the expansion by freezing of the water between its laminæ ; but cross-fracture of a hard homogeneous mass of granite or of greenstone can only be explained by impact or by pressure of the ice-mass in which it was imbedded. The pressure of a glacier or iceberg many thousand tons in weight must be enormous; and the impact of a similar mass drifting at the rate of only three or four miles an hour must be still more tremendous. Any boulder, therefore, adhering to the outside of such a mass, and coming in contact with any submarine ledge, would be readily fractured, and all the more that it was of a flattened or oblong shape, and composed of such crystalline rocks as greenstone or porphyry.

That mere pressure against a resisting mass might be sufficient for the fracture no one knowing the enormous weight of a moving glacier will gainsay, but the fact of the portions lying so closely together and undisturbed is rather against this supposition. On the other hand, the fracture by a drifting iceberg is, perhaps, more easily accounted for, while its stranding and melting away in shallow water would scarcely if at all disturb the position of the fractured portions. On the whole, therefore, we regard these fractured boulders, and especially those lying apart from others, as evidence of floatage by icebergs-the block having been fractured by impact while held in the ice, and then

left with its parts in juxtaposition when the stranded berg had slowly and quietly melted away. To the glacier or land-ice in its long and gradual march from the mountainglen to the sea-shore we can readily ascribe their rounding and smoothing and striation, but for their subsequent fracture and contiguity of parts we must appeal to the drifting and stranding of the iceberg.