386 J. BARRELL FLUVIATILE ORIGIN OF OLD RED SANDSTONE 



sight to overlap and conflict, as, for instance, the presence of gray and 

 green colors and a bituminous composition, together with a striking preva- 

 lence of mud-cracks. Mud-flats covered with sedgy vegetation and black 

 in color may, however, be cracked to depths of as much as a foot without 

 complete drying out and consequent oxidation of the organic matter, the 

 clay having the consistency of a stiff gelatine. Some overlapping and 

 conflict of evidence is, however, to be expected between different horizons. 

 or possibly the same horizon in different basins, since it is known that the 

 Old Bed Sandstone represents a long period of time. There is, therefore, 

 no reason to believe that the climate was strictly uniform throughout. 

 The characteristic oxidation is favored by warmth. The breccias which 

 some geologists have regarded as possibly glacial in origin are in all 

 probabilit}^ torrential deposits from adjacent mountains. The Devonian 

 trees seem to have grown in swamps and do not show growth rings as 

 would be expected in a climate where growth in swamps would be season- 

 ally arrested by cold. The temperatures were therefore probably mild at 

 the lower elevations. The semi-aridity seems to have become more marked 

 in the Upper Devonian, possibly giving rise then to notable proportions 

 of eolian and torrential, as distinguished from fluviatile deposits. 



It may give definiteness to thought to point out how wide-spread are 

 present regions which seem to answer to the climatic limitations as pre- 

 viously determined. An inspection of a chart of the seasonal distribution 

 of rain 40 shows that climates with periodic rains are characteristically 

 continental, the only notable exceptions being the eastern half of North 

 America, Europe north of the Alpine cordillera, western Siberia, and 

 limited equatorial regions. Excluding, however, cold climates and those 

 marked by aridity or high semi-aridity, the general type of the presumed 

 Devonian climates of Scotland is found at present broadly developed over 

 South America east of the Ancles and as far south as Buenos Aires except 

 for two limited regions — the one a narrow equatorial belt, the other the 

 coast of southeastern Brazil. Such warm climates, marked by periodic 

 rainfall and drought, also exist over central Africa, except in the equa- 

 torial Congo basin. They are also developed in Indo-China, more par- 

 ticularly over the southern alluvial basins and deltas. Over the greater 

 portions of these regions the annual rainfall is from 40 to 80 inches, a 

 fairly small amount for the Torrid Zone ; and over considerable portions, 

 especially the indicated portions of Africa and all of Indo-China, the 

 rainfall is so concentrated that from 20 to 30 per cent of the annual 

 precipitation falls within a single month. 



*° See Bartholomew's Atlas of Meteorology, 1899. plate 19. 



