248 C. W. TOMLINSON 



The elastics: minor structural features. — Returning to the clastic 

 sediments, we may draw still further inferences regarding the con- 

 ditions under which they were deposited, from their structural 

 characteristics and mineral composition. Ripple-marks and mud- 

 cracks in the majority of Red Beds sections testify to the prevailing 

 shallowness of the water in which these sediments were laid down. 

 Mud-cracks repeated in layer after layer, as in some parts of the 

 Red Beds, mean complete emergence and at least partial drying, 

 after the deposition of each stratum and before that of the next 

 following. Shallow water means shifting currents, and these too 

 are recorded clearly by cross-bedding in most sandstones of the 

 series, and by rapid variation along the strike in the shaly members 

 as well. We do not have, in the clastic portions of the Red Beds, 

 and seldom do we find in the non-clastic members thereof, the 

 continuity of a single type of sedimentation over wide areas and 

 through long periods of time, which are to be expected in truly 

 subaqueous or marine deposits. Furthermore, imperfect assort- 

 ment, which is one of the universal characteristics of fluviatile 

 deposits, is the rule in the Red Beds. The sandstones are earthy, 

 the shales sandy, the conglomerates gritty, etc. Each of these 

 characteristics is suggestive of subaerial conditions, and the occur- 

 rence of all of them together in the same series, and widely dis- 

 tributed through that series, is conclusive testimony to such an 

 origin. 



The elastics: mineral composition. — From the mineral composi- 

 tion of the clastic sediments we may infer something of the con- 

 ditions of weathering and transportation which preceded their 

 deposition. The high proportion of feldspar in many of the Red 

 Beds shales and sandstones indicates a preponderance of mechanical 

 disintegration over chemical decomposition. The abundance of 

 undecomposed mica flakes in most Red Beds confirms this inter- 

 pretation. Transportation may precede complete decomposition 

 because of exceptional rapidity of disintegration, exceptional slow- 

 ness of decomposition, or both. Rapid disintegration may be 

 caused by such factors as high relief and great daily or seasonal 

 range of temperature; slow decomposition by low rainfall or low 

 temperatures. Low temperatures throughout the year explain 



