24 PROCEEDINGS OF THE INDIANAPOLIS MEETING. 



The coastal sands, or Biloxi sands, as the deposits of the strip immediately adjoin- 

 ing the salt water of Mississippi sound are called, represent now, b}- reason of modern 

 subsidence, a remnant only of their former extent. The strip varies from zero to a 

 width of several miles, stretching, as usually understood, from the Rigolets (or mouth 

 of Pearl river) to Mobile bay. Evidently it once reached out gulfward beyond the 

 present bounding chain of sea islands — that is, if the coast sands be separated from 

 the underlying clays, of which they in fact seem to be only a continuation ; and they 

 are so, but with a sufficient variation in genesis and material to warrant division ; and 

 the deposits were separated by a considerable lapse of time also, though the period 

 was not long enough to permit a change in the fauna of the region affected. Borings 

 at Biloxi, at Pass Christian, on Pearl river near the mouth, and at other places dis- 

 close these Biloxi sands to a thickness of 80 or 100 feet. At Ocean Spring the deposit 

 may be regarded as wanting, while at Scranton the excavation at the railway tank 

 reached the water-bearing clays at 30 feet, and in the overlying sands and marsh mud 

 of this depth were found shells of Venus mei^cena^^ia^ and, it was reported, specimens 

 of the familiar fiddler crab, Gelasinius vocans (or pugilato?^). 



The Biloxi sands consist essentially of thin alternating layers of sand and yellowish 

 brown or blue clay, similar to the deposits now in process of accumulation upon the 

 floor of the sound. In geologic age they evidently represent the beginning of the 

 present or the close of the past. They were unquestionably formed after the mouth of 

 the great river pushed beyond the highlands of Baton Rouge, and when the" passes " 

 were probably about where New Orleans now stands around the great Crescent bend — 

 after the evolution of representatives of lakes Maurepas and Pontchartrain, and when 

 Manchac was the permanent high-water outlet of the main river. If this were not 

 the case; if the Biloxi sands had been deposited at a time when there was a greater 

 divergence of the northern waters eastward and when Manchac was the main outlet, 

 the results would have been essentially similar, but on a grander scale. 



There was, indeed, an era during which the Mississippi erabouched through Man- 

 chac, and during which the action was so much grander and so different in character 

 as to require a different consideration. During this era there were formed what may 

 provisionally and for convenience be called the Pontchartrain clays. As already in- 

 dicated, the method of action was essentially similar to that concerned in the building 

 of the coast sand formation during tho Biloxi period, and similar to that displayed 

 by the comparatively insignificant agency of the Nita crevasse. Hence, although the 

 Pontchartrain clays extend farther inland than the Biloxi sands, or to the edge of the 

 rolling lands of St. Tammany and even to the foot of the gravelly hills of Mississippi 

 and Alabama, as well as up the large estuaries of Pearl and Pascagoula rivers, the 

 blufis, excavations and artesian borings reveal a like sequence of sands and brownish 

 or yellowish-blue clays under a thick coat of clay which forms the water-holding pan 

 of the " pine meadows." 



The topmost layer of the Pontchartrain clays, when tempered by sand or silt, upon 

 the banks of bayous and rivers is not unsuitable for making brick, though it is gen- 

 erally too tenaceous for such use; in fact, it is locally called " pipe clay," from which 

 it really differs ; and it differs also from the brick clay or loam on Amite and Tangi- 

 pahoa rivers, with which it is often confounded. Although this uppermost clay bed 

 is from ten to fifty feet thick in the "pine meadow" region, it contains no fossils, so 

 far as yet discovered ; but it conceals and overlies sands in which excavations have 

 brought to light stumps and logs and, in a few places, marine shells. One general 

 charisteristic of this formation is that it contains abundant water for the supply of 



