prevailed in arctic regions during any past geological age may be regarded as presumptive proof of the existence of a glacial epoch; that is to say, of an epoch during which cold and warm conditions of climate alternated. Keeping these considerations in view, we shall now proceed to. examine briefly what evidence we at present have of the former existence of glacial epochs.

CHAPTER XVIII.

FORMER GLACIAL EPOCHS; GEOLOGICAL EVIDENCE OF.

Cambrian Conglomerate of Islay and North-west of Scotland.-Ice-action in Ayrshire and Wigtownshire during Silurian Period.-Silurian Limestones in Arctic Regions.-Professor Ramsay on Ice-action during Old Red Sandstone Period.-Warm Climate in Arctic Regions during Old Red Sandstone Period.-Professor Geikie and Mr. James Geikie on a Glacial Conglomerate of Lower Carboniferous Age.-Professor Haughton and Professor Dawson on Evidence of Ice-action during Coal Period.-Mr. W. T. Blanford on Glaciation in India during Carboniferous Period.-Carboniferous Formations of Arctic Regions.-Professor Ramsay on Permian Glaciers.-Permian Conglomerate in Arran.-Professor Hull on Boulder Clay of Permian Age.Permian Boulder Clay of Natal.-Oolitic Boulder Conglomerate in Sutherlandshire.-Warm Climate in North Greenland during Oolitic Period.-Mr. Godwin-Austen on Ice-action during Cretaceous Period.-Glacial Conglomerates of Eocene Age in the Alps.-M. Gastaldi on the Ice-transported Limestone Blocks of the Superga.-Professor Heer on the Climate of North Greenland during Miocene Period.

CAMBRIAN PERIOD.

Island of Islay.-Good evidence of ice-action has been observed by Mr. James Thomson, F.G.S., in strata which he believes to be of Cambrian age. At Port Askaig, Island of Islay, below a precipitous cliff of quartzite 70 feet in height, there is a mass of arenaceous talcose schist containing fragments of granite, some angular, but most of them rounded, and of all sizes, from mere particles to large boulders. As there is no granite in the island from which these boulders could have been derived, he justly infers that they must have been transported by the agency of ice. The probability of his conclusion is strengthened by the almost total absence of stratification in the deposit in question.

Brit. Assoc., 1870, p. 88.

North-west of Scotland.-Mr. J. Geikie tells me that much of the Cambrian conglomerate in the north-west of Scotland strongly reminds him of the coarse shingle beds (Alpine diluvium) which so often crowd the old glacial valleys of Switzerland and Northern Italy. In many places the stones of the Cambrian conglomerate have a subangular, blunted shape, like those of the re-arranged moraine débris of Alpine countries.

SILURIAN PERIOD.

Wigtownshire. The possibility of glacial action so far back as the Silurian age has been suggested. In beds of slate and shales in Wigtownshire of Lower Silurian age Mr. J. Carrick Moore found beds of conglomerate of a remarkable character. The fragments generally vary from the size of one inch to a foot in diameter, but in some of the beds, boulders of 3, 4, and even 5 feet in diameter occur. There are no rocks in the neighbourhood from which any of these fragments could have been derived. The matrix of this conglomerate is sometimes a green trappean-looking sandstone of exceeding toughness, and sometimes an indurated sandstone indistinguishable from many common varieties of grey wacke.*

Ayrshire. Mr. James Geikie states that in Glenapp, and near Dalmellington, he found embedded in Lower Silurian strata blocks and boulders from one foot to 5 feet in diameter of gneiss, syenite, granite, &c., none of which belong to rocks of those neighbourhoods.† Similar cases have been found in Galway, Ireland, and at Lisbellaw, south of Enniskillen. In America, Professor Dawson describes Silurian conglomerates with boulders 2 feet in diameter.

Arctic Regions.-The existence of warm inter-glacial periods during that age may be inferred from the fact that in the arctic regions we find widespread masses of Silurian limestones containing encrinites, corals, and mollusca, and other fossil

*Quart. Journ. Geol. Soc., vol. v., p. 10. Phil. Mag. for April, 1865, p. 289. "Great Ice Age," p. 512.

Jukes' "Manual of Geology," p. 421.

remains, for an account of which see Professor Haughton's geological account of the Arctic Archipelago appended to McClintock's "Narrative of Arctic Discoveries."

OLD RED SANDSTONE.

North of England.-According to Professor Ramsay and some other geologists the brecciated, subangular conglomerates and boulder beds of the Old Red Sandstone of Scotland and the North of England are of glacial origin. When these conglomerates and the recent boulder clay come together it is difficult to draw the line of demarcation between them.

Professor Ramsay observed some very remarkable facts in connection with the Old Red Sandstone conglomerates of Kirkby Lonsdale, and Sedburgh, in Westmoreland and Yorkshire. I shall give the results of his observations in his own words.

"The result is, that we have found many stones and blocks distinctly scratched, and on others the ghosts of scratches nearly obliterated by age and chemical action, probably aided by pressure at a time when these rocks were buried under thousands of feet of carboniferous strata. In some cases, however, the markings were probably produced within the body of the rock itself by pressure, accompanied by disturbance of the strata; but in others the longitudinal and cross striations convey the idea of glacial action. The shapes of the stones of these conglomerates, many of which are from 2 to 3 feet long, their flattened sides and subangular edges, together with the confused manner in which they are often arranged (like stones in the drift), have long been enough to convince me of their ice-borne character; and the scratched specimens, when properly investigated, may possibly convince others."†

Isle of Man.-The conglomerate of the Old Red Sandstone in the Isle of Man has been compared by Mr. Cumming to "a consolidated ancient boulder clay." And he remarks, "Was it so that those strange trilobitic-looking fishes of that era had to * See also Quarterly Journal Geological Society, vol. xi., p. 510. †The Reader for August 12, 1865.

endure the buffeting of ice-waves, and to struggle amidst the wreck of ice-floes and the crush of bergs ?"*

Australia.-A conglomerate similar to that of Scotland has been found in Victoria, Australia, by Mr. Selwyn, at several localities. Along the Wild Duck Creek, near Heathcote, and also near the Mia-Mia, Spring Plains, Redesdale, localities in the Colony of Victoria, where it was examined by Messrs. Taylor and Etheridge, Junior, this conglomerate consists of a mixture of granite pebbles and boulders of various colours and textures, porphyries, indurated sandstone, quartz, and a peculiar flint-coloured rock in a matrix of bluish-grey very hard mud-cement.† Rocks similar to the pebbles and blocks composing the conglomerate do not occur in the immediate neighbourhood; and from the curious mixture of large and small angular and waterworn fragments it was conjectured that it might possibly be of glacial origin. Scratched stones were not observed, although a careful examination was made. From similar mud-pebble beds on the Lerderderg River, Victoria, Mr. R. Daintree obtained a few pebbles grooved after the manner of ice-scratched blocks.+

And the existence of a warm condition of climate during the Old Red Sandstone period is evidenced by the fossiliferous limestones of England, Russia, and America. On the banks of the Athabasca River, Rupert-Land, Sir John Richardson found beds of limestone containing Producti, Spiriferi, an Orthis resembling O. resupinata, Terebratula reticularis,§ and a Pieurotomaria, which, in the opinion of the late Dr. Woodward, who examined the specimens, are characteristic of Devonian rocks of Devonshire.

"History of the Isle of Man," p. 86. My colleague, Mr. John Horne, in his "Sketch of the Geology of the Isle of Man," Trans. of Edin. Geol. Soc., vol. ii., part iii., considers this conglomerate to be of Lower Carboniferous age. + See Selwyn, "Phys. Geography and Geology of Victoria." 1866. pp. 1516; Taylor and Etheridge, Geol. Survey Vict., Quarter Sheet 13, N.E.

Report on the Geology of the District of Ballan, Victoria. 1866. p. 11. § Atrypa reticularis.