16i 



HARDWICKE'S SCIENCE-GOSSIP. 



GEOLOGY. 



On the last Stage of the Glacial Period in 

 IMoRTH Britain.— This was the subject of a paper 

 recently read before the Geological Society of 

 London, by T. E. Jaraieson, E.G.S. In this paper 

 the author arranged the Glacial phenomena of 

 Scotland under the following three heads : — 1. The 

 great early glaciation by land-ice (maximum effects 

 of glaciation). 2. The period of glacial marine beds 

 containing remains of Arctic moUusca, when most 

 of the country was covered by the sea. 3. The 

 tiuie of the late glaciers, the special subject of the 

 paper. After expressing himself in opposition to 

 the hypothesis of a great polar ice-cap, the author 

 described this last period as one not of mere local 

 glaciers, but as characterized by a return of a great 

 ice-sheet over nearly the whole of Scotland and 

 Ireland; but he stated that this ice-sheet was 

 probably neither so thick, so extensive, nor so 

 enduring as that of the first period of glaciation, 

 which cleared away everything iu the shape of 

 superficial deposits down to the hard rock. He 

 believed, however, that in the last period the 

 mountains of Scotland and Wales, as well as the 

 Pennine range and the rest of the north of England 

 as far as Derby, were covered with thick ice, which 

 in most parts reached down to the sea, and that 

 extensive snow-beds prevailed over the rest of 

 England. In the summer months the melting of 

 these would give rise to streams of muddy water, 

 and produce the superficial deposits of Brick-earth, 

 Warp, and Loess ; whilst, when the currents were 

 stronger, perhaps from the thaw being unusually 

 rapid, deposits of gravel would be formed. This 

 second ice-sheet would gradually become less and 

 break up into valley-glaciers, which iu their retreat 

 would leave kaims and eskers at low levels, and 

 moraines in the mountain glens. During this tim e 

 no new great submergence of the country took 

 place ; and the last great modifications of the surface 

 were subaerial, and not submarine, the work having 

 been done by frost, rain, and glaciers. In the 

 discussion which followed, Mr. Jeffreys coiisidered 

 that the author's remarks relating to the beds 

 containing Arctic species of mollusca were not quite 

 correct, Fecte7i islandicns has been found in the 

 drift of Scotland, but not in the seas at present 

 surrounding that country. At depths of 30 or 40 

 fathoms many arctic shells in a semifossil state have 

 been dredged, althougli they do not now live in 

 those waters. Mya truncata, a species which lives 

 in very shallow water, has been found in much 

 deeper water in a semi-fossil state. At Port 

 William there is a bed containing arctic species of 

 shells 7 or S feet above the level of the sea. Arctic 

 shells of deep-water species have occurred 200 feet 

 above the sea. Diiferent conditions have existed 

 at different parts of the same seas, altering the 



character of the mollusca. The raising of the 

 sea-beds above the level proper to enable certain 

 mollusca to flourish, would cause them to become 

 extinct. Dr. Carpenter mentioned that cold water 

 may be thrown up into very small depths under 

 certain circumstances. Near Halifax, N.S., the 

 surface-water is tolerably warm, but at no great 

 depth the temperature falls to 35° P. In this case 

 the rotation of the earth causes the cold water from 

 the north to surge up on its western coast. The 

 North Sea is a shallow sea, with a shoal in the 

 middle, and having off the coast of Norway a deep 

 channel which conveys the cold arctic undercurrent ; 

 hence the east side is 10° P. colder than the west 

 side. Local peculiarities of disturbance of tem- 

 perature may thus occur within short distances. 

 Prof, Ramsay remarked that the author was not 

 dealing with wide ocean deposits, but with ice 

 coming down to the sea from the land. He had 

 described certain changes, — a great glacial period, 

 a period of submergence, and a second minor glacial 

 period. Mr. Prestwich maintained that temperature 

 was a most important question in connection with 

 the subject of Mr. Jamieson's paper. The glacial 

 deposits were not formed in deep sea, but in shallow 

 water with shore temperatures. He thought that 

 the paper was very speculative, and remarked that 

 the evidence upon which the opinions expressed 

 were founded was not always given. 



Geology of Clevedon. — In answer to your 

 correspondent " K. L. G." in the May number of 

 Science-Gossip, I beg to state that all the " Bed 

 rocks" of Clevedon, and of theMendip Hills and the 

 Bristol district, belong to the Keuper, or Upper 

 Triassic series. These rocks include the Dolomitic 

 or Magnesian Conglomerate, Red Marl, Sandstone, 

 and occasionally Magnesian Limestone. There are no 

 rocks of Permian age in Somersetshire, and the intro- 

 duction of this name into the county, which has been 

 given in several local pamphlets and guide-books, is 

 due to a misunderstanding of the term Magnesian, as 

 applied to the Dolomitic Conglomerate : it is merely 

 indicative of the matrix or cementing material, and 

 is not used as a mark of age, as with the Magnesian 

 Limestone (Permian) of the northern and midland 

 counties of England. The relation between the 

 various divisions of the Bed rocks themselves is 

 not altogether one of lithological succession, such 

 as is often given in text-books ; namely (in descending 

 order), ]. Marl, 2. Sandstone, 3. Conglomerate, 

 Each of these divisions, as well as the Magnesian 

 Limestones which are seen at Clevedon, also near 

 Bristol and in Glamorganshire, may and do occur 

 anywhere in the Keuper series. The whole history 

 of the Triassic rocks from the oldest beds in South 

 Devon (and some of them are very likely older than 

 the Keuper) shows that Marl, Sandstone, and 

 Conglomerate, or Breccia, were deposited at all 



