Fauna and Stratigraphy of the Stonnberg Series. 467 



delta deposits from the study of the strata requires the demonstra- 

 tion of evidence that both subaerial and subaqueous sediments were 

 deposited - - the subaerial on the landward side of the strand-line 

 and the subaqueous on the seaward (either marine or epicontinental) 

 side. There can be no doubt that during the deposition of the Mol- 

 teno Beds the land which furnished the rock-waste lay to the south 

 and possibly in part to the east of the present outcrop; and, if a true 

 delta had existed, subaqueous deposits would be expected towards the 

 north. It would seem, however, as if the greater part of the features 

 of the beds can be explained better by premising merely terrestrial 

 conditions. At any rate, it can be said that terrestrial conditions 

 will explain the features without calling in the aid of any large body 

 of water. 



Consider the features of aerial deltas in which purely terrestrial 

 deposits are being laid down. Such features vary according to the 

 nature of the climate, whether it be constantly rainy, intermittently 

 rainy, semi-arid, or arid. The late Joseph Barrell worked out the 

 criteria for each type, and to him we owe the following general survey. 



In constantly rainy climates the deposits on piedmont slopes brought 

 down by rivers and floods carry a large amount of humus and forest 

 growth. The soil and subsoil are continually wet and drainage carries 

 away the soluble elements. Thus deposits will contain little or no 

 iron, magnesia, lime, potassium or soda and, on the other hand, will 

 be rich in carbon. They will, on account of the diminished evapor- 

 ation and constant saturation, be white, black, or gray in colour. 

 Coal-seams will be abundant, moisture being necessary for their 

 formation. Barrell says "It may be concluded that the broad as- 

 sociation of carbon with sediments which are thoroughly decomposed 

 and leached throughout is the mark of continuously rainy climates 

 which are tropic or at least warm temperate; with sediments which 

 are imperfectly decomposed and incompletely leached the mark of 

 more or less continuously rainy climates which are in addition cool 

 or cold." 



When the climate is intermittently rainy much of the humus in the 

 soil is yellow or red, and the clays are slightly calcareous. When 

 yellow or brown flood-plain deposits are buried and lithilled the up- 

 stream portions will be found somewhat more arenaceous, varying 

 from red to brown sandstones and usually including red, green and 

 occasionally black shales. Over the terminal land portions of the 

 deposit, the sandstones should be finer-grained and the quantity of 

 shales increase, becoming more gray, green and black. Thick wide- 

 spreading coal beds are impossible : but in occasional swampy areas 



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