AND THE SALT DEPOSIT ON PETITE ANSE ISLAND. 33 



southern portion, was effected by denudation during the Terrace epoch of elevation, 

 when the flood plain of the Mississippi was excavated into the deposits of the 

 Charaplain era of depression. Such portions of the ridge as were sustained by a 

 resisting nucleus (whether cretaceous rock, or heavy drift gravel overlying it), 

 would remain untouched by the denuding currents; while large gaps would be 

 formed where the resistance was less. 



It remains to be shown that the rock-salt mass may, with a considerable degree 

 of probability, be claimed as a cretaceous outlier; and reasoning by exclusion, 

 I think this can be done, by considering successively the formations to Avhich it might 

 be referred. 



As regards the Grand Gidf group, though much impregnated with salts of various 

 kinds, its general character as a fresh or brackish-water formation renders it pecu- 

 liarly ill adapted to the genesis of rock-salt deposits. It is, moreover, a very pre- 

 dominantly littoral formation, whose deep water equivalents appear to be so thin 

 that the Drift currents have in most cases destroyed them. They have not been 

 found in any bore near the coast. The Vicksburg rocks even (which are thinner 

 and of less resisting material in Louisiana than in Mississippi) have been removed 

 in a great measure by the Drift, which in Calcasieu seems to be immediately 

 underlaid by the Jackson group of the Eocene.^ 



But the marine groups of the older Eocene are of such inconsiderable thickness, 

 each so variable in its nature, and so scantily supplied with salt, that to attribute 

 to either of them the formation of so large and pure a mass of rock-salt, seems to 

 involve an utter incongruity. 



Not so with the cretaceous formation that underlies them. Not oidy is salt- 

 water the invariable feature of the cretaceous outcrops of North Louisiana, whence, 

 during the v/ar, heavy supplies were furnished to the blockaded section ; but it is 

 there accompanied by that almost necessary complement, gypsum, which thickens 

 to the soutiiward until, as demonstrated by the Calcasieu bores, it passes beneath 

 the Gulf with the surprising thickness of over six hundred feet. 



It is well known that the end of the cretaceous period on this continent was 

 characterized by a "wholesale" conversion of ocean into inland lakes and dry land. 

 What was, at that time, the condition of the Mexican Gulf basin, we have not the 

 data to determine. But inasmuch as even in early eocene times, water connection 

 still existed between the interior and the Gulf: so of course the same must have 

 been true of the cretaceous inland sea, which by a continuance of elevation inland, 

 was gradually receding toward the Gulf. The existence of the great gypsum for- 

 mation, both in the interior, and beneath the Gulf, argues the concentration and 

 evaporation of a vast amount of sea water as a consequence of the general emergence ; 

 and it is but reasonable that the other chief ingredient — salt — should be found 

 somewhere in connection with the great gypsum beds. And the great rock-salt 

 bed of Petite Anse, now known to exceed seventy feet in thickness, without such 



' Hopkins's researches have proven the Sabinctown equivalent.'^ of the oil-bearhijr rock of Cal- 

 casieu to belong to the Jackson and not (as conjectured by me) to the Vicksburg group. 



5 May, 1S72. 



