TRANSACTIONS

TRANSACTIONS

OF THE

HULL

GEOLOG ICAL

SOCIETY

ABSTRACTS OF LECTURES.

October 25th, 1894.

" THE GEOLOGY AND PHYSICAL GEOGRAPHY OF CENTRAL AFRICA."

By the Senior Vice-President of the Society, the Rev. E. MAULE COLE M.A., F.G S.

In his introductory remarks the lecturer said it was a mistake to suppose that the geography of Central Africa was entirely unknown to the ancients. As a matter of fact the existence of several 0fthe great Central lakes was known, and their position was set down with some amount of precision by both Herodotus and Ptolomy. Ancient maps circa 150 A.D., show the Nile issuing from two lakes and Portuguese maps of the 15 Century also show some of the lakes ; but maps of Africa made at the beginning of the present century are quite blank in the vast central regions.

Dealing with Central Africa as now re-discovered by Livingstone, Stanley, and Cameron, the lecturer said that the great water-parting of the area was so situated that a single rainstorm might flow in three directions -- by the Nile, northwards; by the Congo, westwards ; and to the East, by the Zambesi. The great lakes of Central Africa, like all lakes, are being filled up rapidly by the detritus brought down by their tributary streams. The waters of the Victoria Nyanza are found to dry up at the rate of three inches per year. The South end of this lake is a gentle slope, covered with mud flats, and saline deposits, extending for thirty miles, showing the previous extent of the waters. All round the Albert Edward Nyanza are similar deposits. Lake Tanganyika occupies the western side of a great hollow, 600 miles by 300 in breadth. The lake itself, 400 miles long by 20 wide, lies along a line of great disturbance. At one point, near the middle of the western shore of the lake, there is a V shaped gap in the mountain barrier. At this point Stanley found a river running into the lake, but when Cameron visited the place he found the water of the lake had risen so as to be level with the depression and forming a morass. Subsequently Hore found a stream four feet deep running violently out of the lake. In 1888 the lake had fallen 18 feet, but the water was still running out of the depression, and the barrier of detritus was cleared out down to the solid rock. Thus in quite modern times the rainfall has overcome the evaporation and filled the lake up to the brim. The water of Lake Tanganyika is fresh, but Lake Rukwa in the same district is salt, and the water has now shrunk 40 miles from the old margin.

With regard to the volcanoes of Central Africa, the lecturer said that the only knowledge we possessed was that gleaned from the casual reports of travellers who were not geologists. Between the Albert and Albert Edward Nyanza Lakes, was Mount Gordon Bennett, in the crater of which live a people almost white. Many other extinct craters had been found, in some of which there were crater lakes.

December 20th, 1894.

THE ENGLISH LAKE DISTRICT.

PROF. A. HARKER, M.A., F.G.S., (Cambridge University).

Mr. Harker began by describing briefly the oldest rocks of the district, the Skiddaw Slates, which occupy a large area, especially in the Northern part. In these rocks four or five fossiliferous zones have been recognised of ages ranging from Tremadoc through Arenig to Llandeilo. Owing to the great disturbance, the stratigraphical relations are rather obscure, but it seems probable that all these fossils belong to the Upper part of the Skiddaw Slates, which may therefore possibly include the equivalents of zones lower in the Cambrian, not yet identified in the North of England. Clifton Ward estimated the total thickness at 10,000 to 12,000 feet, but in view of the folding and faulting of the Strata, we cannot yet attach much weight to such measurements.

Succeeding the Skiddaw Slates is the great succession of volcanic rocks sometimes named the Borrowdale Series, constituting the Central part of the district and forming most of the higher mountains. These rocks were probably formed under water, and some of the tufts or ashes are well banded; but, except near the top of the series, there is no admixture of detritus with the volcanic material. The intercalated fossiliferous sediments seen in the partly contemporaneous volcanic series of North Wales are here wanting. Any decisive evidence regarding the exact sites of actual volcanic centres is also yet to be obtained. The succession is a varied one. So far as it is yet deciphered, the earliest rocks seem to be, as in numerous other volcanic districts, lavas of intermediate composition which we may term andesites. To these succeed basic lavas (basalts without olivine) covering considerable tracts of country. Next comes a thick group of fragmental accumulations (tufts and breccias), well seen in some of the main mountain-groups such as those of Sea Fell, Bow Fell, and Helvellyn. Finally we have acid rocks, chiefly lavas (rhyolites), and the upper flows of these are interbedded with the succeeding Coniston Limestone.

The exact correlation of the Coniston Limestone has been discussed by Mr. Mart, who has shown that it is divisible into at least four distinct groups, each of which has its distinctive fauna and its equivalents in other areas. There is not in this district, as in some others, a stratigraphical break between the Ordovician (Lower Silurian of the Geological Survey) and the Silurian proper. The lowest division of the latter, the Stockdale Shales, equivalent to the Llandovery and Tarannon beds of Wales, is an example of the extremely detailed subdivision which can be introduced by means of careful paleontological study. Using chiefly the characteristic species of graptolites, Mr. Marr and Prof. Nicholson have recognised some fourteen distinct zones, some not more than a few inches thick but still persistent over a large area, and corresponding with zones containing the same fossils in the South of Scotland and in Sweden. Of the succeeding groups, the Coniston Flags, Coniston Grits and Bannisdale Slates correspond with the Wenlock and lower Ludlow of the Welsh Border, while the Kirkby Moor Flags are the equivalent of the Upper Ludlow.

The lecturer described the nature of the remarkable system of crust-movements which followed the Silurian in this district, the effects of which are seen in the folding and over-thrust faulting of the Lower Palaeozoic rocks and a great unconformity representing roughly the missing Old Red Sandstone Strata. The Carboniferous and New Red rocks which border the district were only briefly referred to. In Mesozoic times the area was elevated, probably by slow degrees, into a dome-like form, and the result of this is still seen in the radiate drainage system of the district; a system which was initiated in consequence of this dome, but, by the gradual stripping off of the Carboniferous and newer rocks, became superposed upon the Lower Palaeozoic core of the district to which i has no direct relation.

The circumstances attending the glaciation of the district and the origin of the lakes, which are now its most striking feature, were only briefly touched upon in the concluding part of the lecture.

January 17th, 1895.

PHOTOGRAPHY AS A MEANS TO THE STUDY OF GEOLOGY.

J. E. Bedford ESQ., F.G.S.

In his introductory remarks Mr. Bedford alluded to the great advantage photography placed in the hands of the modern astronomer and geologist, and in geology especially with regard to sections on the coast and elsewhere which were constantly changing their aspect. The lecturer however said he had not come to give an elaborate paper on the subject but simply to show the Hull Society what had already been practically accomplished by a few West Riding geologists in the direction of geological photography. He then caused to be put on the screen a fine series of views illustrating a wide range of formations throughout the British Islands, including the Giant s Causeway, Fingal's Cave, Port Erin, Snowdon, Llanberis, Bournemouth, the Cornish Coast, Dunkeld, Arbroath, Skye, Malham, Dove Dale, Chee Tor, Flamborough, Filey, etc.

"LIFE IN TIME" March 4th, 1895.

E. BOARDMAN, ESQ.

The lecturer first alluded to the immense thickness of the stratified rocks, estimated by Lyell at 100,000 feet, and taking the opinion of some geologists of the older school that the rate of deposition might average one foot in one thousand years, made the age of the oldest stratified rocks one hundred million years. Mr. Boardman then discussed the evidence we have as to change in climate in the geological past, and the various alleged causes of the same.

Finally the lecturer reviewed the paleontological evidence as to the first appearance and development of organic Life on the Earth --shewing that the fossil forms in the strata from Palaeozoic times till now indicated a gradual passage from lower to higher forms of Life.