84 E. W. Hilgard—Geology of Lower Louisiana. 
been observed with certainty. Even the older strata underly- 7 
ing the blue stump clay have been observed at a few points 7 
only, viz: by the Delta Survey in the bed of the Mississippi river — 
at Bonnet Carré and Carrollton, near New Orleans; at the lat- — 
ter city itself, in the boring of wells ; at Salt Point, on Bayou 
Salé ; and on the coast of Mississippi Sound. 
The strata penetrated in the borings at New Orleans are con- 
sidered by Sir Chas. Lyell as Delta deposits. But according 
to my examination, they are almost throughout demonstrably 
of marine origin, and while the species they contain are mostly — 
(not all) now known to be living on the Gulf coast, yet the — 
prevalence of species is very different from that now observed 
near the mouths of the Mississippi. In this respect, the fauna f 
of these strata shows a great analogy to those described as — 
Pliocene by Tuomey and Holmes, occurring on the Carolina 
coast, 
It is most probable that the rock-salt of Petite Anse will be — 
found when pierced, to be imbe in the equivalents of the fF 
deposits penetrated at New Orleans and Bayou Salé, and of — 
corresponding, probably early quaternary age, anterior to the 
_ drift or its southern representative, the Orange Sand. 
Origin and extent of the salt deposit. 
The absence of layers of the usual impurities of rock-salt, | 
especially of gypsum, has induced Dr. Gdssmann to suppose 
that it is not the result of the evaporation of sea-water, but 
owes its formation to crystallization from the purer brine of | 
salt springs. t 
Our knowledge of the facts is still too limited to render @ | 
discussion of this point very profitable. Ina very deep lagoon, | 
withdrawn from the influx of the tides after the brine had | 
mass of pure salt, as in the present case ; while the salts of 
mother-waters would naturally have been washed away from 
low wells on the coast, beneath the « stump clay. 
Upon any of the foregoing co 5 oye calling into play @ 
variety not 
