AvGisT 7, 1903.] 



SCIENCE. 



181 



tion in your issue of July IT seems to call for 

 some comments on my part, since I am orig- 

 inally responsible both for the name and 

 definition of that formation as such. The 

 situation appears to me to be this, that while 

 Smith and Aldrich bring what seems to be 

 irrefragable proof that what I have described 

 as the Grand Gulf formation is newer than 

 any well-defined Oligocenc. Dall lays stress 

 upon the reported dipping of the Grand Gulf 

 under Oligocene strata in Florida and Texas, 

 and suggests that the ' Grand Gulf Sand- 

 stone ' of Wailes is the original genuine Grand 

 Gulf, with which certain clays and lignitifer- 

 ous strata have subsequently been, perhaps 

 wrongfully, associated. 



Now as a matter of fact, Wailes used the 

 term Grand Gulf sandstone merely as a litho- 

 logical designation, not as the name of a for- 

 mation; and while he correlates with it the 

 sandstones of some other localities, he de- 

 scribes under the same general heading other 

 light-colored sandstones, belonging, respect- 

 ively, to the Burstone and to the Lafayette. 

 On the other hand, he distinguishes by the 

 name of ' Davion rock ' the undoubted equiv- 

 alent of the Grand Gulf at and below Fort 

 Adams, Miss. According to usage, I might 

 have adopted any other name for the forma- 

 tion as a whole, since Wailes failed adequately 

 to characterize it. But as I found the ex- 

 posure at Grand Gulf to be a really general- 

 ized and representative one, I thought it best 

 to apply Wailes' lithological designation to 

 the formation as a whole. It rests with me, 

 therefore, to justify my correlation of the 

 sandstone formation of the central portion of 

 the Mississippi embayment with the clay for- 

 mations from the Pascagoula to the Sabine, 

 leaving to others the proof of identity beyond 

 these limits. 



In the absence of specificially identifiable 

 fossils, it is not easy for the field geologist to 

 satisfy the critics at home as to the correct- 

 ness of his perception of that often indefin- 

 able something called fades, which is never- 

 theless oftentimes as cogent as specific 

 identities of fossils, especially with the 

 modern view of species. Even in the absence 



of the chalcedonization which characterizes 

 the genuine (and rare) Grand Gulf ' petrified 

 sandstone,' the sandstones of the Grand (lulf 

 age can not easily be mistaken in the field for 

 any of those occurring in other horizons in 

 the southwest. From the Bayou Anacoco on 

 the Sabine, via Bayou Funne Louis to Har- 

 risonburg on the Washita, and from Grand 

 Gulf to Kaleigh, Miss., its fades, both litho- 

 logically and stratigraphieally, is unmistak- 

 able, although the chalcedonized rock facies 

 is mostly absent and everywhere, except at 

 Grand Gulf, quite subordinate; mostly in thin 

 ledges or lenticular masses. The sandstones, 

 mostly rather soft, tend to cleave vertically 

 rather than horizontally, and are markedly 

 poor in mica. 



The clays occurring interstratified with 

 sandstone layers and ledges are sometimes, 

 but not always, as characteristic as the sand- 

 stone itself, but much more so when occurring 

 independently in large masses, as is especially 

 the case on Pearl River and its tributaries. 

 Doubtless the physical analysis of these massy 

 clays, which range in tint from blue and grreen 

 to reddish-gray, would be found to indicate 

 the characteristics which render them so stri- 

 kingly dissimilar to those of other formations 

 of the southwest, whether older or later. One 

 of their characteristics is the almost total 

 absence of mica, which is so abundant in the 

 earlier Tertiary as well as in the later Lafay- 

 ette and is there conducive to the prevalent 

 lamination ; indicating apparently a deriva- 

 tion from different sources, not so far inland 

 as to reach the micaceous metamorphics. 

 The clays mostly contain a very large propor- 

 tion of fine siliceous silt, so that while plastic 

 they are not usually very adhesive. They are 

 (especially in Louisiana) not infrequently 

 consolidated into a soft siliceous claystone. 



A highly interesting feature of the Grand 

 Gulf clays is the local occurrence of calcare- 

 ous concretions and veins, which I think may 

 fairly be attributed to the presence, locally, 

 of a rather copious fauna of shells, whose 

 shape has been destroyed by maceration. In 

 the calcareous clays underlying the ' Anacoco 

 Prairie ' in western Louisiana, many of the 



