CLIMATE AND TERRESTRIAL DEPOSITS 271 



The section of the Ganges delta given by the Calcutta borehole 

 shows no trace of marine deposits, but on the other hand the proofs 

 of land surfaces in the form of ancient swamp deposits, even in this 

 marginal portion of the delta, were encountered only at two levels 

 in the 481 feet of the boring.^ Scattered vegetable matter and the 

 bones of terrestrial mammals and fluviatile reptiles which were found, 

 while suggestive of continental deposition, may possibly occur in 

 off-shore marine deposits and alone do not conclusively demonstrate 

 the continental origin. Farther inland, at Umballa, on the water- 

 shed of the Indo-Gangetic plain, a bore-hole 701 feet deep passed 

 through alternations of sand and clay, the colors usually red or brown 

 but with some clays blue and black. In places the bore-hole encoun- 

 tered a few pebbles and bowlders, but no mention is made of organic 

 remains, which according to Medlicott and Blanford occur but rarely 

 in the alluvial formations of the Gangetic plain. ^ Numerous layers 

 of Kankar (concretionary strata of calcium carbonate) suggest the 

 previous existence at this place of the semiaridity which now prevails 

 in that region. The Mississippi delta shows much of the same char- 

 acteristics as that of the Ganges at Calcutta. Consequently, where 

 sands and clays or their consolidated representatives constitute a 

 formation with no trace of marine fossils but possessing even fragmen- 

 tary remains of land life, it is to be concluded with high probability, 

 if no other evidence overweighs the decision, that the entire formation 

 is continental and, further, if no positive marks of other climatic 

 conditions are evident, that it was probably formed on a river flood 

 plain under the intermittently rainy climates, which, though at times 

 diminished and again magnified in importance, have yet existed unin- 

 terruptedly through all geologic time and formed those shifting zones 

 within which the chemical activities of the atmosphere and the bio- 

 logic forces of terrestrial evolution have found their fields for fullest 

 action. 



EFFECTS OF SEMTARID CLIMATES 



Chemical and structural characteristics. — Semiarid climates are 

 those where irrigation is usually necessary for the maturing of crops 



1 Medlicott and Blanford, A Manual of the Geology of India, Part I, 1879, pp. 

 397-400. 



2 Op. cit., pp. 401, 402. 



