« PreviousContinue »
ence of submarine currents. Sir James Anderson says, in a letter to me—" Perhaps the most marked experience we have had of currents at great depths was in the case of the Falmouth cable, near Gibraltar. At 500 fathoms the wire was ground like the edge of a razor, and we had to abandon it and lay a new cable well in shore. Captain Nares, of the surveying ship 'Nemesis,' I think, with tangles could get no specimen of the bottom whatever, and he thinks he got sufficient evidence to prove the existence of a perfect swirl at that depth." Again we have it on the evidence of Prof. Verrill that the deposits under the Gulf Stream, even at great depths, do not correspond with the "Challenger" experiences. Large blocks of sandy clay were brought up from 1,060 fathoms, and large masses of hard but sticky greenish blue clay from 1,168 fathoms, and between 2,000 and 3,000 fathoms in all the 10 localities "Globigerina Ooze" occurred. The Eed clay distinguishing such depths, according to the "Challenger" investigations, was not once met with.*
According to the late M. Milne Edwards, the "Talisman" soundings showed the bottom of the Sargassum Sea to be formed of "a thick layer of a very fine mud of a pumice nature, covering fragments of pumice and volcanic rocks." Between the Azores and France the sea bed "is carpeted throughout this region with a thick white mud composed almost exclusively of Globigerinae, and covering pumice deposits and fragments of various kinds of rocks. Some of these rocks brought up in our nets bore the impress of fossils, amongst others of Trilobites. But what more surprised us was to find, at a distance of over 700 miles
• Marine Fauna and Deep-Sea Deposits off the Southern Coast of New England.—Ainer. Jour, of Science, Nov., 1884.
from the European coast, pebbles polished and striated by the action of ice." The sea bed stretching westward of Morocco and the Sahara was found to be " extremely uniform, no longer presenting those rugged reliefs that had so impeded our operations on the coast of Spain."* Prof. Sars says the extensive depression between Norway, the Faroe Islands, and Iceland appears everywhere below 1,000 fathoms "to consist of a very peculiar loose, but very adhesive, exceedingly light, nearly greyish white clay, which is very strongly calcareous, and on being washed or passed through a sieve, appears to consist almost exclusively of shells of a little low organism belonging to the Foraminifera Biloculina." "The Biloculina clay of the cold area contains a greater quantity of lime than the Globigerina clay of the Atlantict At a distance of from 70 to 140 English miles from the coast of Norway, at the edge of the submarine barrier, in depths of from 300 to 100 fathoms, the bottom is generally hard and stony." "Numerous rolled stones, whose smooth rounded forms and worn edges clearly enough shew that they had at one time been subjected to the powerful action of ice, lie here strewn on the sometimes very uneven bottom, consisting of solid rock, and prevent the dredge from acting properly."I This submarine platform as a rule rises somewhat towards its outer edge before it slopes towards the great depths lying beyond it, and simultaneously assumes, as in the well-known case of the Storegg, a hard, stony character. "At the first sounding, when we went out from Husoe,
• The " Talisman" Expedition.—Nature, Dec. 27, 1883. t Mr. Gwyn Jeffreys says the "Porcupine" dredgings gave from 50 to 60 per cent, ot carbonate of lime.—Nature, Feb. 3, 1881, p. 326.
; The Norwegian North Sea Expedition of 1876.—Sature, March 8, 1877, p. 412, vol. xv.
we struck this edge at about 140 English miles distance from the coast. The bottom, which before had everywhere appeared to be soft, suddenly at a depth of 221 fathoms became hard and stony, and retained this character even after it had sloped about 50 fathoms down towards the deep sea lying beyond."*
The difference in the nature of the submarine plateau off the coast of Norway and the belt of shallow soundings off the mouth of the Amazons, and which shallow belts more or less distinguish several of the coast lines of continents, especially near the mouths of great rivers, appears to be that the former is a submarine extension of the continental framework of the land, while the latter is the effect of accumulated sediment. The deeply serrated coast line of Norway indicates a considerable subsidence, for these great fjords, profound in depth, running up miles into the heart of the land, have undoubtedly been formed by the denudation of subaerial agents at a level much above the sea line. Under these circumstances we find the rugged nature of the country continuing far seawards under the rolling waves of the Atlantic. In the case of great continental rivers discharging into the open ocean or deep seas, the soundings are shallower and comparatively regular up to the edge of the great oceanic slope.
In conclusion, I am pretty well convinced from the reasons already given, namely, the great age and permanency of the river basins—such as the Amazons, which has certainly existed since Tertiary times—the depth of alluvial and other modern deposits at the mouths of most great rivers, the existence of extensive platforms about their mouths with uniform shallow water over them, the absence of rocks and presence of
• Ibid.—Nature, Vol. xv., p. 436.
modern sediments, that deposits are now taking place in the sea thousands of feet thick, and parallel to those, we know, have occurred over and over again in geological history. These deposits are, in my view, submarine extensions of the true deltas, which will make themselves felt in future geological history.
These considerations open up many geological questions of surpassing interest, of which we have hardly yet touched the fringe. The further we penetrate the unexplored regions of physical geology the larger appears the extent of country before us. Deeply sensible as I am of the slight and imperfect character of the sketch presented to you to-night, I trust that it may not be entirely barren, but may be found to contain some ideas that will germinate and yield further fruit.
ON BITUMEN IN THE PALAEOZOIC ROCKS
By Charles Ricketts, M.D., F.G.S.
After the field-meeting held last summer in the neighbourhood of the Wrekin, several members, under the guidance of Dr. Callaway, F.G.S., visited Haughmond Hill. Approaching it from Upton Magna Station, the party entered a quarry situated at its base, near Downton. It was found to consist of Longmynd shales and slate, full of small shrinkage joints; the rock being broken up into small pieces, with each of the crevices thus formed filled with mineral pitch.
At Nills Hill, Pontesbury, the quartzite band, forming the northern extremity of the Stiper Stone range, contains many cracks and fissures, small cavities, and interstices between quartz pebbles in bands of conglomerate, filled with a similar substance.
In the calcite derived from the mineral veins in the Bog Mine, Shelve, instances have been observed of the interstices between the crystals being filled with bituminous matter.
Sir B. I. Murchison directed attention to the exudation of bitumen from the rocks at Haughmond Hill and likewise at Pitchford, considering that "if these substances were formed out of vegetable or animal matter, we can refer to seaweeds or minute worms only as the sources of them." * He appears to have entirely overlooked the character of the strata with which these rocks are associated. Coal-measure strata overlying Longmynd rocks occur about a quarter of a mile S.W. of the quarry in Haughmond Hill; the small area of Longmynd strata at Pitchford is entirely surrounded by Coal-measures. Coal pits exist at Pontesbury, a quarter of a mile northward of the Stiper Stone quartzite of Nills Hill; and the same locality is within five miles of the Bog Mine, which is also only seven miles from Le Botwood, a Coal-measure district to the east of the Stiper Stones. With the near proximity of coal in each of these localities, and the extreme probability that Coal-measures once extended over the whole, it is requisite to consider whether the cracks and fissures found in the rocks were formed before the Carboniferous strata were deposited; also the probability of the bitumen being derived from the decomposition of the vegetable matter contained in the coal strata once situated above them, but which have been removed by denudation, from which it percolated through the strata to the fissures before alluded to.
• " Siluria," Third Edition, p. 25.