336 SCIENCE PROGRESS 



interpretation is favoured by the marked sedimentary rhythm which is described 

 in the series by Crampton and Carruthers. The views of these authors may be 

 illustrated by a quotation (Caithness Memoir, p. 103): "Lake Orcadie may thus 

 be pictured as one of several sheets of very shallow water varying in number, 

 position, and communication with one another, with a permanent drainage system 

 depending on the direction of crustal warping and the age of the accumulations 

 in the basin. 



"With semi-arid surroundings and a warm climate, its waters supported 

 abundant plant and animal life, and were subject to extensive advances and 

 retreats from variations in the water-level due to silting and periodic seasonal 

 changes, or from bodily migrations induced by warping of the basin of deposit. 

 At one time extensive lakes, at another endless mud-flats and stagnant lagoons. 

 At first, semi-desert shallow valleys and half-buried mountain peaks with fans and 

 flood plains of ephemeral streams ; later, extensive lakes and alluvial deposits ; 

 and, at last, a wind-swept plain of sand and wandering water-courses." 



This view is nearly indistinguishable from that formulated by Prof. Barrell, 

 of flood-plain conditions which, however, do not exclude subordinate marginal 

 semi-desert phases and impersistent lacustrine intercalations ; but it almost 

 entirely disposes of the conception of great permanent lakes such as those 

 postulated by Sir A. Geikie. Nevertheless, in the discussion preceding this 

 statement there is much insistence upon sedimentation in Lake Orcadie ; but the 

 reader is often left in doubt in the case of a particular formation, whether it is 

 considered due to flood-plain deposition marginal to the lake, or to deposition in 

 the lake itself. In fact, no clear distinction is made between the fluviatile and 

 lacustrine phases of deposition. 



The palaeontological evidence is no less favourable to the hypothesis of 

 fluviatile origin of the Old Red Sandstone than the lithological and structural 

 evidence. The fossils consist of fish, eurypterids, Crustacea {Estheria), and plants, 

 a biota decidedly more continental than marine. The presence of Estheria 

 membranacea is especially important, as its fossil as well as its present-day 

 associations stamp the landlocked and freshwater habitat of this organism. 



The absence of unequivocally marine fossils throughout a great series of strata 

 creates a presumption against, although it does not entirely negative, its marine 

 origin ; but when this negative evidence is reinforced by the presence of tre- 

 mendously thick, widespread conglomerates, and by the prevalence of marks of 

 subaerial exposure throughout a great vertical thickness and areal extent of strata, 

 the presumption of continental origin becomes irresistible. The theory of marine 

 origin of the Old Red Sandstone has been upheld on the ground that fishes of 

 the same genera have been found in the marine Upper Silurian and Devonian 

 as well as in the Old Red Sandstone ; but this does no more than emphasise the 

 fact that fishes (also, probably, eurypterids and Estheria) may adapt themselves 

 to either freshwater or marine habitats, and may thus be entombed in the 

 respective deposits. It has also been held that the interdigitation of beds carrying 

 marine fossils in the Russian Old Red Sandstone, and the presence of " marine " 

 beds in the immediately preceding Downtonian of the south of Scotland, proves 

 the marine origin of the Scottish Old Red Sandstone. The Russian area, 

 however, is far from Scotland, and is not proved to contain the distinctive 

 Scottish facies of the Old Red Sandstone. It probably represents the site of 

 deltaic accumulations on the margin of the Old Red Northland, in which some 

 interfingering of the seaward and landward deposits took place. Prof. Barrell 

 throws doubt on the marine nature of the Downtonian fauna, which consists of 



