ESSAY-REVIEWS 335 



marine shingle seems to have an upper limit of ioo ft. (H. E. Gregory, "The 

 Formation and Distribution of Fluviatile and Marine Gravels," Amer.Jour. Sti., 

 191 5, 39, 487-508). These facts are well supported by the evidence of un- 

 questionably marine conglomerates in the geological record, which seldom exceed 

 100 ft. in thickness. On the other hand, gravels consisting of well-worn boulders 

 and pebbles are now accumulating to depths of over a thousand feet along the 

 Himalayan foothills, as well as along other mountain ranges, and are closely 

 comparable in thickness and character to the Old Red Sandstone conglomerates. 

 Stream-laid gravels may be carried far and wide over a flood-plain, and thus form 

 very extensive deposits ; but subaqueous gravels, marine or lacustrine, are of very 

 limited lateral extent, since the currents and waves are unable to carry coarse 

 material far from the shore, and tend to deposit it in layers parallel to the coast. 

 Hence thick and extensive conglomerates are far more likely to be of fluviatile 

 than of marine or lacustrine origin. 



On river flood-plains, which may be desiccated in the dry season, marks of 

 exposure to the atmosphere are abundant, especially mud-cracks (sun-cracks, 

 desiccation-cracks). Not only may these marks have a wide areal distribution, 

 but the conditions of their formation may persist throughout the deposition of a 

 great thickness of sediment. Within tidemarks, however, the area upon which 

 they may be developed is extremely limited, and being swept by tides twice a 

 day, the chances against the preservation of the marks are very great. If some 

 happy chance ensures their preservation, exposure-marks will then be confined to 

 thin beds on a continually rising horizon, and will be of restricted areal extent. 

 The seasonal oscillation of the level of a lake may provide better and longer 

 exposed mud-surfaces for the development of exposure-marks, but these marks 

 cannot be continued through any great thickness of strata. It is clear, therefore, 

 that a widespread areal and vertical distribution of marks of subaerial exposure 

 in a series of strata is evidence of their fluviatile origin. 



All accounts of the Lower Old Red Sandstone in the typical area of " Lake 

 Caledonia" agree in the description of huge masses of conglomerate consisting 

 mainly of water-worn pebbles of quartzite and vein quartz derived from the 

 adjacent Highland Schists. Macnair says {Geology and Scenery of the Grampians^ 

 1908, vol. ii. p. 2) they extend from Stonehaven on the north-east to Arran and 

 Kintyre on the south-west of Scotland, and gives the thicknesses of various 

 beds as 300, 3,300, and 1,000 ft. on the north-western side of the main syncline, 

 and 1,100 ft. on the opposite side. Campbell {op. tit.) gives 6,900 ft. as the 

 thickness of the Dunnottar conglomerate at the base of the Lower Old Red 

 Sandstone of Kincardineshire. Conglomerates of this enormous thickness and 

 extent can only have been deposited under fluviatile conditions along the fronts 

 of great mountain ranges, as is indeed recognised by Campbell. 



The Middle or Orcadian Old Red Sandstone contains no great con- 

 glomerate deposits. It is characterised by a great flagstone series containing 

 numerous fish remains, with arkoses and mudstones of subaerial origin at the 

 base, and a thick sandstone formation (John o' Groat's Sandstone) at the top. 

 The flagstone group, according to Prof. Barrell, was deposited on the lowest 

 portions of river flood-plains, where more or less continuous swamp conditions 

 prevented the aeration and desiccation which gave rise in earlier times to red 

 sediments. These conditions favoured the preservation of organic matter and 

 the maintenance of the iron in a low state of oxidation, giving dominantly grey, 

 green, or blue colours to the sediments. Hence the flagstone series appears to 

 represent the consolidated sands, silts, and muds of river alluvia ; and this 



