Page:The Habitat of the Eurypterida.djvu/192

184 the material which filled up the mid-Scottish basin or "Lake Caledonia," for it was hemmed in on the west by a narrow ring of hills separating it from "Lake Lorne" in North Argylshire, and on the south by hills along an east-west line through the Firth of Forth, and on the north by the Highlands which were the source of the 16,000 feet of sediments deposited in Lake Orcadie, while to the east the sea covered France. The only other source would be a mountain chain in the present English Channel, but the objections to this are obvious. A natural question that arises often in reading Geikie's monograph, and one which Macnair and Reid most pertinently ask is how outliers of conglomerates on the tops of high mountains, in the very regions which were supposed to have been lake barriers, are to be accounted for. Geikie has proposed that perhaps they represent old fiord-like indentations in the shoreline. This explanation will not serve, however, when such outliers are found on what must have been the very centre of the ridge between Lakes Orcadie and Caledonia, such, for instance, as Macnair and Reid mention at Mealfourvonie just north of Loch Ness in Inverness where an outlier is found 2284 feet above sea-level, and at Tomintoul in Banff, and Rhynie in Aberdeen. The outliers in all parts of Scotland indicate that the deposit was essentially continuous, though varying in lithological character and origin from place to place.

(e) Structural features. The cross-bedding, ripple marks, and other structural features that are cited by some authors as indicative of marine littoral conditions of sedimentation, by others as lacustrine littoral, will be considered below under the third theory of the origin of the Old Red sandstone (p. 189).

(2) Faunal. Attention should be called to certain erroneous lines of argument that have been used and which fall down because based on false premises. For instance, it is impossible to prove that the Old Red sandstone eurypterids were marine by saying that the Siluric ones were and that therefore the Devonic ones of the same genera must also be. First it must be proved that the Siluric eurypterids were marine. To quote once more from Macnair and Reid: "We have . . . . seen no reason assigned why Eurypterids and Placoderms of the same genera, which are marine in the late Upper Silurian, and fishes of the same genera and species which are equally marine in the Devonian of Russia and Central Europe, as well as in the Devonian of North America, should be termed equivocally marine in the Old Red sandstone" (160, 219). It may be remarked that the