(Agassiz, ' Etudes sur les Glaciers.') On the grooves and strife on the surfaces of rocks Mr. Jukes remarks (' Popular Physical Geology,' p. 280), in considering the former action of ice, here alluded to, " In addition to ice however, Mr. Mallet has acutely pointed out that mauy of these appearances may be duo to what he somewhat inappropriately terms mud glaciers,' by which he means the eliding forward or slipping of great masses of clay, mud, or sand, charged with pebbles and boulders, along the inland surfaces of rock, either as the land rose from the sea, or when they were subsequently loosened by the action of rain and other water." The examination of the boulders of certain parts of the II imalayau range, and the application of the theory of glacier movement to explain their distribution, has led Dr. Joseph D. Hooker to suggest some modifications of that theory in respect of those localities. The banks of the Great litmgeet river, near its junction with the Kulhait, consist of mica-slate, cumbered, in a deep gorge through which the river flows, with enormous boulders of that rock, of clay-slate, and of granite, some fully 10 feet in diameter. But the latter rock is not common at eleva tions below 10,000 feet, whereas the absolute elevation of the river here is only 1840 feet ; it is not easy therefore to account for their present position. "They have been transported," Dr. llooker infers, " from a considerable distance in the interior of the lofty valley to the north, and have descended not lees than 8000 feet, and travelled fully fifteen miles in a straight line, or perhaps forty along the river bed.
It may be supposed that moraines have transported them to 8000 feet (the lowest limit of apparent moraines), and the power of river water carried them further ; if so, the rivers must have been of much greater volume formerly than they are now." Another explanation was re quired by the enormous fractured boulders of gneiss frequent over the whole of the mountain Mona Lepcha, in the Sikkim Himalaya, at elevations of from 7000 to 11,000 feet. Contrary to those mentioned above, they were of the same material as the rock in situ, and as unac countable in their. origin, by received theory, as the loose blocks on the Dorjiling and Sinehul spurs to the south, at similar altitudes, often cresting narrow ridges. Dr. Hooker measured one angular detached block, 40 feet high, resting on a steep narrow shoulder of the spur, in a position to which it was impossible it could have rolled ; "and it is equally difficult to suppose," he observes, " that glacial [glacier] ice deposited it 4000 feet above the bottom of the gorge, except we con clude the valley to have been filled with ice to that depth." A third modification of or addition to the glacier theory of boulder distribu tion, is pointed to by the locality of tire Jongri spur of the great mountain Kinehinjunga [OrtoLocy], over which are scattered blocks of gneiss, many 20 feet in diameter. " It is not possible to account
for the transport and deposit of these boulders by glaciers of the ordi nary form, namely, by a stream of ice following the course of a valley ; and we are forced to speculate upon the possibility of ice having capped the whole spur, and moved downwards, transporting blocks from the prominences on various parts of the,,spur." Himalayan Journals,' vol. i., pp. 242, 253, 288.
The entire subject of the transport of erratic blocks, and of the hypotheses which have been framed to account for it, has been criti cally examined by Mr. W. Hopkins, in a paper ' On the Elevation and Denudation of the District of the Lakes of Cumberland and Westmore land,' in the Quart. Journ. of the Geol. Soc.,' vol. iv.
The great mass of diluvium from the Cumbrian mountains, already alluded to, which covers the surface of Lancashire, rests on nothing more recent than the new red sandstone, and Mr. Hopkins conceives that its transport might have begun with the elevatory movements which disturbed that formation, when the surface of the present moun tainous district began to rise permanently above the surface of the ocean, and the valleys began to be formed. The spreading out of diluvial matter—that term being now adopted in its broad, physical sense—may be regarded as the necessary consequence of wide general currents, and that this has been the agency by which the mass of diluvium in question has been transported to its present locality and position, does not admit, in Mr. Hopkins's opinion, of the smallest doubt. He accounts for the existence of currents diverging from the centre of the district by a repetition of paroxysmal elevations of from 100 to 200 feet; and, affirming the entire adequacy of this cause to transport all the erratic blocks derived from that region, concludes that such has been the agency by which that transport has actually been effected. He rejects the iceberg theory in its application to the ease investigated, but conceives that floating ice may probably have been the most efficient agent in transporting the larger blocks of colder regions from their original localities. Mr. Hopkins's paper may be studied with great advantage with reference to the whole subject of the distribution of boulders and of the northern drift.
We do not propose to investigate any of the hypotheses above stated, preceding the facts and reasoning we have derived from Dr. Hooker and Mr. Hopkins. Geologists have been remarkable for eagerly adopting and as easily abandoning most of them; and others might have been added merely as beacons to be avoided. It may be proper however to point out three things which may be useful to remember in further prosecuting this subject, and which, indeed, have neither been forgotten nor neglected, by the geologist last named.