12 ON THE GEOLOGY OF LOWER LOUISIANA 



doubtless, are the " strata" of limestone mentioned by Thomassy' as occmTins; at 

 Belle Isle, and whose " shattered" condition he attributes to volcanic explosions. 



Towards the east end of the bluff, stratum No. 5 descends to tide-level by a 

 sudden curve (whether owing to a basin-shape or a landslide, does not clearly appear). 

 Here the ferrugino-calcareous concretions begin to assume definite shapes, and for 

 a few yards there is a kind of clay marl, filled with casts and much decayed shells 

 of fresh-Avater mollusks. With the exception of rfPahidina, resembling (but not 

 identical with) P. stthptirpnrea of Sat, I found it impossible to preserve any 

 sjiecimens of these in a condition fit for subsequent determination. But the shape 

 and hinge enabled me to recognize at least four species of Unio, a Cydas, the thin 

 shell of an Anodonta, and a longitudinal section of a Potamidcs. There are also 

 some ferruginous casts of plants, badly preserved. 



Doubtless, the lime of the nodules has been derived from the shells of fossils, 

 which have been destroyed by a process of dissolution and recrystallization, from 

 which but a few spots have remained exempt. 



There is not, on this island, another exposure showing more than the surface 

 loam with greenish clay beneath. Remains of the Mastodon have, in times past, 

 been found on the island; but I have been unable to ascertain the precise locality. 

 The stump clay has, however, been observed on several sides, in digging trenches 

 for drainage in the marsh. 



WEEKS' ISLAND. 



A ride of six miles carries us from CCte Blanche to Weeks' Island, which is 

 reached from the southeast, by a causeway a mile in length, traversing the almost 

 bottomless Cyprus ]\Iort swamp, swarming with alligators. On all other sides the 

 island is surrounded by sea-marsh. Its form is nearly circular, area about two 

 thousand three hundred acres ; highest elevation, according to Thomassy, one 

 hundred and sixty feet above tide-level. Its surface is not as unbroken as that of 

 Cote Blanche; both on the outer slopes and in the centre, there are deep ravines 

 Avhich render the formations accessible. The soil and original vegetation are the 

 same as at Cote Blanche. 



The ravines in the central portion exhibit on their sides, in the most unmistakable 

 manner, the characteristic stratification and materials of the Orange Sand, or 

 Stratified Drift formation — chiefly the brownish-red, semi-indurate sand, consisting 

 of rounded quartz grains incrusted with iron rust, together Avith a greater or less 

 proportion of grains of white pipe-clay. Here and there, also, layers and fanciful 

 concretions of the well-known brown sandstone of Mississippi; Avhile at some of 

 the lower points, the pebble-bed appears, with its usual variety of Avcll-worn rocks 

 from higher latitudes, and some large and rough ones of Grand Gulf sandstone. 

 The stratification lines are horizontal on the large, but wavy and discordant on tlie 

 small scale, with flakes of white pipe-clay to mark them. So far as lithological and 



' Geol. prat., p. 80. 



