284 ANNALS NEW YOIiK ACADE31Y OF SCIENCES 



One must concede that a hypothesis might be framed wliich would ac- 

 count satisfactorily for many conditions in a given locality and still be 

 based on an impossibility. Before considering the applicability of the 

 delta theory to Decazeville, one must ascertain whether or not the funda- 

 mental conditions demanded by that hypothesis existed in the region 

 under consideration. 



The Decazeville basin is very small; the earliest deposits were laid 

 down very soon after the depression was formed, for the rocks belong to 

 one subdivision of the Coal Measures and there is no evidence that rocks 

 of greater age intervene between them and the Archean. The character 

 of the Coal Measures shows that the streams in the earlier period were 

 short and torrential; that even during the greater part of Campagnac 

 deposition they had cut back only a few miles, had not a Avide fan of 

 tributaries, but were still more or less torrential and flowed in narrow 

 valleys such as are seen now in and around the basin. They emptied 

 into an at most wholly insignificant body of water, so small that during 

 floods such as have been imagined by some adherents of the transport doc- 

 trine everything except coarse stuff would be carried directly to the outlet 

 at Firmy, and the finer materials would be deposited outside of the basin. 



Under such conditions, all citation of phenomena observed on deltas of 

 the Mississippi, Nile, Ganges, La Plata or even of the Ehone, would be 

 irrelevant. Streams flowing hundreds or thousands of miles througli 

 broad plains and carrying for the most part fine material in their lower 

 reaches, along with trees gathered from immense areas by undermining 

 of banks cannot be utilized to illustrate what a three or even 10 mile tor- 

 rent, flowing over tough mica schist or microgranulite and inclosed in a 

 narrow valley would or could or might do. Nor may the deposit at the 

 mouth of a petty stream emptying into a pond be eom])ared in any sense 

 with the delta of a vast river. In fact, the term "delta" as applied to 

 the theory seems to have been chosen without full consideration. The 

 writer has not been able to discover in the ]\Iontassiege area of the Com- 

 mentry basin any evidence of a true delta. The deposit is rather a great 

 dejection cone, largely under water perhaps, with no doubt temporary 

 delta conditions during excessive floods, but during most of the time 

 trenched by only one waterway. It might be well in discussing the con- 

 ditions to avoid the term delta as not wholly appropriate, and to think- 

 instead of broad dejection cones such as one sees in the Rhone valley 

 between Viege and Martigny. At least one of those cones is so mucli 

 like the earliest Auzits deposit in form and extent as to make not un- 

 reasonable the suggestion that the latter began as a subaerial deposit. 



Another matter may be noticed in a preliminary way. The delta doc- 



