18 ON TUE GEOLOGY OE LOWER LOUISIANA 



wliere the more coherent strata immediately overlie the solid Drift nucleus, the 

 dips are moderate or nil ; while these same strata near the circumference of the 

 island show a dip of twenty-five and more degrees, not explicable without some 

 subsequent disturbance. Now, we know that around these islands, as elsewhere in 

 the littoral belt, there is a floor stratum of blue clay with cypress stumps and muck, 

 whose gradual conversion into the more compact form of lignite Csuch as we find at 

 Cute Blanche) would of necessity be accompanied by a considerable diminution of 

 bulk, followed by a corresponding subsidence of the superincumbent deposits. 



Some of the clayey strata, moreover, exhibit a phenomenon somewhat resembling 

 the movement of a glacier down a mountain slope ; a prominent example of which 

 occurs on the eastern slope of the hill on which Judge Avery's residence stands. 

 Although distinctly stratified, this clay (which is filled with fossil impressions) has 

 an uncontrollable tendency to cleave into angularly conchoidal forms, the cleavage 

 planes being so smooth as to glisten when dry. When wet, of course they are so 

 exceedingly slippery that even large masses give way under slight pressure, and it 

 thus happens that the whole stratum, so far as exposed, is now bodily moving down 

 hill; having, for example, completely overflowed a deposit of brickbats, and yet 

 preserved in a great measure its stratification. In consequence of this disturbance, 

 it is very difficult to ascertain the true thickness and position of this stratum, which 

 in aU respects greatly resembles No. 5 of the Cote Blanche section, as well as the 

 corresponding deposits of Weeks' Island. It contains an abundance of calcareous 

 concretions, mostly with a ferruginous nucleus, and also purely ferruginous ones. 

 Numerous impressions of plants occur, among them the leaves of grasses and reeds, 

 of Salix, Mijrica cerifera, Quercus virens, aquatica, and phellos — verified by com- 

 parison on the spot; as the specimens are excessively perishable. 



Somewhat lower down, we find in it badly preserved shells of Paludhia, of several 

 species of Unto, and a Oijclas. In consequence of the great tendency of the matrix 

 to cleave, it has been impossible to preserve specimens for determination ; it would 

 be difficult to determine them specifically even on the spot. The Paludina is 

 clearly, however, identical with that found at Cute Blanche. 



This is the only fossiliferous locality I have observed in this formation at Petite 

 Anse, although the equivalent of the fossiliferous clay occurs at several other points, 

 containing, however, only calcareous concretions. On the -opposite slope, i. e. a 

 short distance northwest of Judge Avery's house, the following profile, typical of 

 the exterior slopes of the island, is shown in a gully. The strata are nearly hori- 

 zontal at the highest point, but near the foot of the hill acquire a dip of twenty- 

 five degrees northwest ; while a few hundred yards off, the blue clay with cypress 

 stumps forms the bed of Bayou Petite Anse. 



