32 ox THE GEOLOGY OP LOWER LOUISIANA 



inanifostly transported in rapid, shallow water, shall be found beneath the level of 

 the Gulf.' 



ORIGIN OF THE FIVE ISLANDS, AND AGE OF THE SALT DEPOSIT. 



It is only by the light of the observations and considerations given above, bearing 

 on the geological history of the Gulf region, that wo can usefully approach the 

 problem of the age of the Salt deposit of Petite Anse. 



Since the lowest (clay and pebble) strata of the Stratified Drift are found over- 

 lying the rock-salt mass, its age is at once removed beyond the luaits of tlie 

 quaternary j^eriod. And, in fact, unless we resort to legerdemain hypotheses of 

 local upheavals, salt springs miraculously evaporated in loco, or a lagoon arrange- 

 ment resembling that of the " salt-plantations," it is not easy to refer the formatico. 

 of so large and pure a mass of salt even to a period as modern as the tertiary, witli 

 its very slow and regular emergences, and frequent alternations of character. Its 

 existence where it is, however, is scarcely more anomalous than that of the chain 

 of five elevations whereof Petite Anse fo>rms one, on a coast otherwise inonotonously 

 flat for seven hundred miles each way. 



It is but fair to assume, a priori, that the chain of islands and the rock-salt r.iass 

 are genetically related, until the contrary be proven. 



In seeking for an adequate cause, there are two circumstances v.lucli render the 

 position of these islands exceptional, viz: — 



1. They lie diagonally across the shortest line by which the coutinental waters 

 could reach the sea, riglit in tlie axis of the Mississippi embayment. 



2. They lie also directly in the line of a cretaceous axis of upheaval, indicated 

 by a group of outliers traversing the State in a south-southeast direction; the last 

 characteristic one occurring only sixty miles (on a direct line) inland of Petite Anse. 



Given a solid, resisting cretaceous nucleus: the accumulation on it of Drift 

 materials, the deposition of the Port Hudson series at a higher level here than 

 elsewhere, and the greater resistance to denudation during the ^Terrace epoch is 

 readily accounted for. 



It might be thought that these elevations represented the bar, as it were, of the 

 great central Drift current of the Mississippi Valley, where its velocity was checked 

 by the sea. Indeed, the large-sized pebbles of high northern origin found on Petite 

 Anse, prove that the main pebble stream was not altogether deflected to the south- 

 east (as is the case with the Mississippi and its bayous, in the lower part of the 

 river's course) ; and it is quite credible that the predominant accumulation of coarse 

 materials in this main axis may, in a great measure, account for the unusual resist- 

 ance to denudation offered by these islands. For it can hardly be doubted that 

 the whole of the Attakapas prairies was originally covered with deposit to a similar 

 extent as is still the case in their northern portion, and at Port Hudson; and that 

 the removal of the higher, silty materials down to the "stump clays," in the 



* I. e. four hundred aud filly feut, so far as at present observed in the Calcasieu bores. 



