D. W. Langdon — Geology of Mon Louis Island. 23 1 



only by the thickest sections between crossed nicols, while 

 thin sections show gray of the first order. The hardness of 

 mordenite is about 3. How gives 5 for his mineral. While it is 

 difficult to ascertain the exact hardness of a species occurring 

 in such small brittle crystals it was certainly not so hard as 5, 

 and 3 is believed to be more correct. Before the blow-pipe 

 mordenite does not exfoliate ; it gives off its water readily, 

 practically without changing its form, and melts with some 

 difficulty to a white enamel. 



The writer desires at this place to express his obligations to 

 Mr. Arnold Hague of the IT. S. Geological Survey, to whose 

 kindness the collection of this mineral was due, and to Prof. 

 S. L. Penfield for valuable assistance and advice. 



Mineralogical Laboratory, Sheffield Scientific School, April 14th, 1890. 



Art. XXXI.- 1 - Geology of Mon Louis Island, Mobile Bay ; 

 by Daniel W. Langdon, Jr. 



In 1855 Tuomey* was handed some fossiliferous, ferruginous 

 sandstone from the western shore of Mobile Bay, containing 

 impressions of Cardium magnum, Ostrea Virginica and &Mod- 

 iola resembling M. demissa, but was unable to fix definitely 

 the locality. In 1885 Dr. Geo. H. Taylor, of Mobile, gave the 

 writer a small box of shells obtained from the mud dredged in 

 the channel of Mobile Bay, some ten miles from the Gulf. 

 From their physical appearance they were supposed to be fossil 

 — perhaps Pliocene or even Miocene, and with the idea of es- 

 tablishing this fact they were submitted to Mr. T. H. Aldrich, 

 of Blocton, Ala., who in turn forwarded them for identification 

 to Mr. W. H. Dall, of the National Museum. Mr. Dall decided 

 that they were recent shells now living in the deeper waters of 

 the Gulf and probably washed in the bay by submarine currents. 

 Some time later Dr. Taylor submitted another lot of shells con- 

 tained in the same matrix, an impalpable blue mud, and said to 

 have been found on the Mon Louis Island, some fifteen miles be- 

 low Mobile. A trip to the island proved its identity with Tuo- 

 mey's locality, " Yellow Jack " being a Creole patriarch whose 

 descendants still inhabit Mon Louis. As was stated by Tuomey's 

 informant, this fossiliferous stratum was found to be about 

 three feet above mean tide, and was clearly the oxidized and 

 lithified phase of the shell-bearing blue mud occurring at 

 various elevations along the coast of the island to within about 

 four miles of the Gulf, and found in the dredged channel of 

 the Bay. These blue mud deposits are sometimes filled with 



* 2d Bien. Report on Geol. of Ala., pp. 149-150, 1859. 



