336 STUDIES FOR STUDENTS 



by erosion ; but it is doubtful to what extent such deep downbreakings 

 as those of the mediterranean basins are ever restored to the surface 

 of the land, and their deeper deposits consequently opened to obser- 

 vation, the most favorable chance being where the region becomes 

 involved in a later mountain revolution. Whether or not the deposits 

 of the shallow basins are continental or marine will depend upon the 

 vigor of stream-erosion, the rapidity of subsidence, the breadth and 

 height of the surrounding lands, and nearness or distance from the 

 sea. As epeirogenic and orogenic movements have been intermittent 

 and variable in nature, the continental or marine infillings of such 

 basins will thus have varied largely through geological time. 



DELTA DEPOSITS 



A delta may be divided into two chief portions, one of which, above 

 the water, forms a low land surface, usually fertile and densely in- 

 habited ; another portion being deposited beneath the sea, and build- 

 ing forward the front of the delta. As is well known, borings in the 

 Mississippi, Ganges, and Po deltas have revealed fresh- water fossils and 

 beds of vegetable matter at some hundreds of feet beneath the present 

 level of the sea. Such facts have led to the view that large delta sur- 

 faces are frequently regions of subsidence, and that they may be 

 maintained above the sea-level by the continual deposit of river 

 material. Thus it is seen that delta formations are divided into por- 

 tions which are continental, littoral, and marine. 



The possibly subaerial delta origin of certain ancient formations 

 has been long since suggested; to cite a single instance, as far back 

 as 1886, Bonney, in his presidential address before the British Asso- 

 ciation, concludes that the English Bunter is probably a subaerial 

 delta formation, analogous to the Siwalik deposits of India.' Not- 

 withstanding such instances, however, it will probably be admitted 

 by most that, in interpreting the mechanical deposits of previous ages, 

 especially where these are thick, barren, and suggestive of discharge 

 at the mouths of large rivers, usually no adequate discussion has been 

 given as to the possible intermingling of subaerial and submarine por- 

 tions. The section at one place may represent wholly the land- 

 surface deposits; at another, the wholly off-shore zone; and at still 



I Report of the British Association for theAdvancement of Science, 1886, p. 618. 



