of the Lafayette Formation. 391 



The substance of this gravel is by far predominantly chert 

 of Subcarboniferous age, less frequently siliceous sandstone and 

 quartzite ; but the corals and crinoicls, as well as occasionally 

 the trilobites of the older Paleozoic rocks are always more or 

 less represented. ~No calcareous rocks have been found any- 

 where ; but in and near the main channel of the Mississippi, 

 pebbles of crystalline rocks — granite, syenite, gneiss, mica-schist, 

 as well as dioritic greenstone and eruptives — are not uncom- 

 mon, though always much rounded, usually small but occa- 

 sionally of several pounds weight, and usually also much 

 decayed. A similar assortment of crystalline rocks was ob- 

 served by me at Petite Anse island, on the Gulf shore, in the 

 direct line of the axis of the Mississippi valley. 



"While the size of the pebbles commonly ranges from that of 

 a walnut to that of a fist, there occur occasionally bowlders of 

 considerable weight, up to several hundreds of pounds, of the 

 same rock material as the smaller gravel. 



The structure of the formation is extremely irregular. It 

 may broadly be said to be approximately horizontally bedded 

 or stratified, without having any well-defined general dip, save 

 in that it conforms to that of the general surface of the country 

 to southward. In its upper portions it is sometimes wholly 

 structureless in profiles of as much as twenty feet, when the 

 material is usually somewhat clayey and constitutes a sandy 

 loam. Most commonly however the bedding-lines are con- 

 spicuous and very complex, showing the flow-and-plunge struc- 

 ture of sand-bars, often alternating with that of beaches, and 

 then again, particularly on the more consistently continuous 

 stratification lines, the peculiar wavy structure which belongs 

 to the flood-deposits of valleys. The bedding and stratification 

 lines are commonly conspicuously marked by the ferruginous 

 coloring which pervades more or less the whole formation and 

 doubtless forms an important landmark in its history. 



In this irregularity of stratification and bedding the formation 

 differs pointedly from the bulk of the Tertiary formations that 

 have preceded it on the same areas ; although locally, cross- 

 bedding occurs in these as in all formations of sandy materials. 

 But in the Lafayette beds the structure resulting from deposi- 

 tion in flowing and violently agitated waters is all but universal. 



In its stratigraphic relations to other formations the " Orange 

 Sand" of the Mississippi embayment shows a prevalent uncon- 

 formity with both the older and later beds with which it is in con- 

 tact. It has been deposited on a deeply eroded or sculptured 

 surface, and has itself suffered deep erosion before the 

 deposition of the succeeding deposits. Yet inasmuch as in its 

 formation it has largely drawn upon the materials of the sub- 

 jacent beds, apparent conformity frequently exists where these 



