CONGLOMERATES AND BRECCIAS.

Ir is easy to assert-and the assertion seems true-that conglomerates are compacted and cemented gravels, and that breccias have arisen from a similar consolidation of rubbly or angular fragments; but it is not so easy to comprehend by what agencies such heterogeneous masses as the conglomerates of the Old Red Sandstone and Permian formations have been aggregated, or by what processes the fragments of brecciated rocks were first broken up and subsequently brought together without undergoing a much greater amount of attrition. Indeed, the formation of conglomeratic masses several hundred feet in thickness, and composed of blocks and boulders of all sizes, is a very intricate problem, and demands more consideration than it has yet received from working geologists. Let us glance for a while at some of these difficulties, and try to indicate, if we can, the way to their removal.

And first let us see how far the physical operations of the present day throw any light on the question; for unless we appeal to this source, any explanation that may be offered can at best be little better than guess-work and conjecture. Along the exposed shores of all free-flowing seas—that is, seas subjected to wave and tidal action

gravel occurs in greater or less abundance, and in pebbles from the size of a pea to that of a bomb-shell;* and it is not difficult to understand how, by the infiltration of lime or of iron, or of calcareous and ferruginous muds, such gravel might be converted into conglomerate. Indeed, in the so-called "littoral concretes"—that is, masses of cemented beach-gravels-we see the conversion actually in process, and there is no difficulty in accounting for the formation of many of the older conglomerates by a similar method. Again, in ice-locked seas, or seas that are for the greater portion of the year covered by ice, and where the fragments torn from the cliffs by frost are not subjected to attrition and rounding by the waves, we can as readily understand that the consolidation of these shingly fragments would result in a breccia; and how, also, such rubble and shingle carried away by shore-ice might be piled up on some distant coast and converted into a brecciated conglomerate. Miles of shingly talus were noticed by Dr Kane during his arctic voyage, not only at the existing sea-level, but in raised beaches of various altitudes; and Dease and Simpson, in passing Point Barrow, were witnesses of the manner in which beach-ice, laden with shingle, could during heavy storms pile up the material high above ordinary tides, and for leagues along - shore. And farther, we can likewise understand how-on a coast like that of Greenland, where glaciers bring down blocks and boulders to the shore, and where storms and ice-floes lift and impact such miscellaneous debris above water-mark-how, in course of time, it might be converted into a rough rubbly conglomerate alto

* One of the finest examples of a gravel beach, if indeed not the finest in Europe, is that of the Chesil Bank, stretching for miles alongshore, and out from the mainland to Portland Island, and composed of pebbles so gradually assorted that fishermen and smugglers, it is said, can determine their place on the bank even during night by the size of the pebbles.

gether different from that produced by the ordinary action of waves and tides.

By no other process than one or other, or all, of the preceding, are pebbly, gravelly, and bouldery masses aggregated at the present day, whether in tropical, temperate, or polar regions; and we may rely that by no other means were conglomeratic rocks produced in former ages. In trying, therefore, to account for the conglomerates and breccias of the stratified formations, we must first examine the nature of the constituent fragments, the amount of attrition they have undergone, and, above all, the manner in which they are piled together, and then see how far the facts can be explained by appealing to existing operations. By so doing there is little difficulty in accounting for the formation of Post-Tertiary river-drifts, raised-beach gravels, or littoral gravels and shingles, whether loosely aggregated or compacted into concretes and conglomerates; and there is usually as little difficulty in explaining the origin and accumulation of Tertiary conglomerates and puddingstones. The only masses connected with these periods which present real difficulties are the kaimes, eskars, osars, and similar so-called "drifts" that are spread so widely over the higher latitudes of either hemisphere. These, so long as the ordinary operations of water are appealed to, seem inexplicable. Their great magnitude, their peculiar positions, the heterogeneous nature of their composition - angular blocks, boulders, gravel, and nests of sand, and the irregular manner in which these ingredients are arranged-point obviously to moving forces of gigantic power; and no forces in nature with which we are acquainted seem equal to the result save moving ice, whether dropping its burden as bergs and floes, or leaving its moraines as the glacier and ice-foot. At this conclusion all, or nearly all, geologists have arrived; and though the subsequent action of rain and rivers has so

greatly altered the original features of these masses that we cannot always perceive the precise nature of the producing ice-force, still everything within them-the huge angular blocks, the smoothed and striated boulders, and the pellmell aggregation of coarser and finer material-points to conditions essentially and unmistakably glacial.

Again, when the conglomeratic or rather pebbly beds of the Greensand, Trias, or Coal formation are examined, they present so little that is at variance with ordinary shoreaction, that we ascribe them at once to this source, whether consisting of thick-bedded masses, or merely of pebbles scattered here and there through the sandstone strata. The thicker beds are but the long-accumulated gravels of the exposed sea-shores of these periods, while the solitary or sparsely-scattered pebbles may have been raised by storms, floated by sea-weeds, or borne by other means of occasional transport. We see nothing in their composition at variance with this explanation-nothing that may not have taken place along the shores of any free-flowing sea of the present day; and as to the presence of solitary pebbles in sandstone, or even occasional boulders in chalk, we perceive every winter how pebbles may be floated by the attachment of sea-weeds and cast on soft sandy shores, and how boulders may be dropped by melting icebergs on the chalky ooze of the Atlantic thousands of miles away from their parent habitats. But when we come to the bouldery conglomerates of the Old Red Sandstone-such as those exposed in the bold sea-cliffs between Stonehaven and Bervie, in the Forfarshire hills of Turin and Finhaven, or along the shores of Rothesay-and to the breccias of the Permian or New Red Sandstone as exposed in the Abberley and Malvern hills, or along the wasting sea cliffs of South Devon, then we meet with a composition so heterogeneous (angular blocks, boulders, rounded pebbles, and interlaminated sandstones), and with

an arrangement so heterogeneous, that the ordinary operations of waves, tides, and currents are altogether unequal to the explanation.

With regard to the conglomerates of the Old Red Sandstone, while its constituent fragments are evidently derived from all the older rocks-quartzes, granites, porphyries, gneisses, grits, slates, and mica-schists-these fragments occur so unequally water-worn, so unequally assorted as to size, and so unequally arranged as to form, that no power of moving water, whether in waves, tides, or currents, can be appealed to for their accumulation. Here we have lying side by side rounded boulders and pebbles of quartz with sharply-angular blocks of grit, porphyry, and gneiss—there we have blocks more than a ton in weight imbedded in pebbles less than a walnut; here we have blocks and boulders lying on their flattened sides, and, jammed among them, slabs on their thinnest edges-there we have the hardest quartz worn into the smoothest pebbles alongside fragments of mica-schist that have scarcely lost their fractured corners ; here a patch of the finest-grained sandstone imbedded among the roughest blocks and boulders; and generally throughout, large and small, rounded and angular, soft and hard, thrown pell-mell together without the slightest reference to form or to gravity. Clearly such an arrangement could never have been produced by the waves of a free-flowing sea, however fierce its storms or gigantic its breakers. We may have gravel of all sizes, shingle of all shapes, and boulders bulkier than those of the "boulder beach" of Appledore, in North Devon; but we have never rounded and sharplyangular fragments together, never fine sand interlaminated with shingly fragments, never flattened slabs indiscriminately standing on edge or lying on their sides, and never hard rock fragments commingled with soft ones which a few days of wave-action would reduce to impalpable sediment. The