1 82 THE HABITAT OF THE EURYPTERIDA 



Old Red Sandstone are equally marine "(160, 221). The chief objec- 

 tions to this theory of marine denudation continuing from the begin- 

 ning of Upper Siluric to the end of Lower Devonic time, fall into 

 three groups: (1) Tectonic. The tectonic relations between the Old 

 Red sandstone and the underlying rocks show that there was pro- 

 found folding at the end of the Siluric, followed by a long period of 

 erosion before the earliest Old Red sediments were deposited; there- 

 fore the two series are not conformable as claimed by Macnair and 

 Reid. (See further p. 173 above). (2) Liihologic. (See below, sec- 

 tions (d), p. 182, and (1), (2), (3) on p. 189). (3) Faunal. (See 

 below, section (b), p. 191). 



(c) Salt indicative of marine deposition. The argument that the 

 presence of a salt-bearing stratum in the Old Red at one locality is 

 undoubted evidence of the marine origin of that bed, is of no value 

 unless supported by critical data on the chemical composition of the 

 salt and associated salts if any are present, and on the organic con-. 

 tent. Too much is now known concerning the continental origin 

 of many and perhaps the larger number of past and present salt 

 deposits for anyone to claim that the sea was always or even commonly 

 the immediate source of the material. Macnair and Reid would 

 make the presence of the salt band an a priori reason for its marine 

 origin, for they say: "We find in the Moray Firth area a large stratum 

 of yellow saliferous sandstone, interbedded with shales containing 

 remains of Old Red sandstone fishes .... and we think that 

 but one conclusion alone can be drawn therefrom — that the formation 

 and its contained fish remains were marine" (160, 221). This type 

 of reasoning is delightfully ingenuous and one that is met with fre- 

 quently; while the authors do not explicitly state any reason why 

 the salt is marine, the reader yet receives the impression that the 

 presence of fish remains carries a strong presumption, and thus we 

 have the pleasing circle: "The salt is marine because associated with 

 fish, and the fish are marine because found in bands interbedded 

 with salt-bearing sandstones." This whole argument would fall to 

 the ground were anyone to show that the fish were fluviatile, or that 

 the salt could have some other origin. 



(d) Thickness of deposits. The recurrence in the same place of 

 thick boulder and pebble conglomerates interbedded with sandstone 

 and shales, all being dominantly red and showing a complete absence 

 of unequivocal marine fossils such as brachiopods, molluscs, crinoids, 

 and trilobites, and amounting in thickness to many thousands of feet 



