382 Correspondence—Mr. Searles V. Wood. 
In this paper, which forms a sequel to their description of the 
Glaciation of the Shetland Isles, the authors, after sketching the geo- 
logical structure of Orkney, proceeded to discuss the glacial pheno- 
mena. From an examination of the various striated surfaces, they 
inferred that the ice which glaciated Orkney must have crossed the 
islands in a north-westerly direction, from the North Sea to the 
Atlantic. They showed that the dispersal of the stones in the Boulder- 
clay completely substantiates this conclusion; for in Westray this 
deposit contains blocks of red sandstone derived from the island of 
Eda, while in Shapincha blocks of slaggy diabase, occurring in situ 
on the south-east shore, are found in the Boulder-clay of the north- 
west of the island. Again, on the mainland, blocks of the coarse 
siliceous sandstones which cross the island from Inganess to Orplin 
are met with in the Boulder-clay between Honton Head and the 
Loch of Slennis. 
Moreover, they discovered in the Boulder-clay the following 
rocks, which are foreign to the island:—chalk, chalk-flints, oolitie 
limestone, oolitic breccia, dark limestone of Calciferous-sandstone 
age, quartzites, gneiss, etc., some of which closely resemble the 
representatives of these formations on the east of Scotland, and have 
doubtless been derived from thence. From this they infer that, 
while Shetland was glaciated by the Scandinavian mer de glace, 
Orkney was glaciated by the Scotch ice-sheet, the respective ice- 
sheets having coalesced on the floor of the North Sea and moved in a 
north-westerly direction towards the Atlantic. 
They also found abundant fragments of marine shells in most of 
the Boulder-clay sections, which are smoothed and striated precisely 
like the stones in that deposit. They conclude that these organisms 
lived in the North Sea prior to the great extension of the ice, and 
that their remains were commingled with the moraine profonde as 
the ice-sheet crept over the ocean-bed. From the marked absence of 
shell-fragments in the Shetland Boulder-clay, they are inclined to 
beleve that much of the present sea-floor round that group of islands 
formed dry land during the climax of glacial cold. 
CORRES PON DENCH. 
SSSR Sera 
DR. CROLL’S ECCENTRICITY THEORY. 
Sir,—My letter in your March Number has not elicited any 
explanation from Dr. Croll, but has one from Mr. Wallace; in reply 
to whom I would observe that Mr. Croll not only admits that the 
eccentricity would be inoperative on climate but for its causing a 
diversion of the ocean currents, but he endeavours in great detail to 
show that the most important of all these currents, and the one on 
which the difference between the climates of Western Hurope and 
Eastern North America depends, the Gulf Stream, was totally 
diverted and turned southwards along the coast of South America so 
as not to enter the North Atlantic at all. 
The modification of Mr. Croll’s theory which Mr. Wallace now 
offers we shall be better able to understand when his book appears ; 
but to speak of the influence of the Polar extension of North 
