IOWA acadp:my of scie:nces. 39 



8. Sands of Tertiary age, false bedded, party-colored, coarse or fine, banded 

 often with drab, red or white pipe clay, or the last may be in pockets or lenses. 

 These sands are {jenerally loose, but in certain localities they have metamorphosed 

 into a very hard, trlassy (juartzite. The areas of metamorphism are linearly dis- 

 tributed over many square miles, but are confined chiefly to the west side of the 

 ridge. Silicified woods are found in this member at many localities, but none has 

 yet been discovered in the metamorphosed portions. 



4. Drab, blue and black clays of Eocene Tertiary age, horizontally stratified, 

 occasionally fossiliterous, the fossils being chiefly the leaves of deciduous trees. 

 These clays contain rare beds of lignite of small extent and erratic vertical distri- 

 bution. Moreover, the clays are commonly gypsiferous and are further character- 

 ized by abundant small plates of rauscovite in the cleavage planes. Silicified wood 

 was seen at a single locality, on Cache River. 



The absence of fossils in nearly all the members of the Arkansas Tertiary ren- 

 ders necessary their distinction upon lithological and structural data. The large 

 masses of silicified wood in the upper members of the series are the only organic 

 forms known above the Eocene clays. If in any way these silicified woods may be 

 genetically connected with the lignite beds a means of correlation will not cer- 

 tainly be had, but the fact may sometime possess taxonomic value. Studies made 

 in Eastern Arkansas seem to show that all or nearly all of the silicified woods of 

 the Tertiary sands and gravel beds are derived in some manner from the underly- 

 ing beds of lignite. In many places whole tree trunks, stumps standing in place, 

 or large fragments of silicified wood occur so related to lignite deposits as to show 

 that they are derived therefrom. In the northwestern portion of Greene county, 

 on the west side of Crowley's Ridge, are masses of wood partly in the form of lig- 

 nite and partly silicified. The lignitized part is buried in Eocene clays; the silici- 

 fied ends are buried in Eocene Tertiary tands. It would appear that in this case, 

 before the sands were eroded away, the portion of the trunk which had been buried 

 therein was subjected to the action of waters containing silica in solution and the 

 lignitic matter was replaced by silica. 



The silica is. of course, all present as secondary quartz, is often massive but, 

 also, frequently crystallized. Especially is holocrystalline quartz abundant in 

 specimens of wood that were partially decayed when the older lignification process 

 began. In the drusy cavities of such lignite are found large numbers of perfect 

 and rather large quartz crystals. These are often, in some specimens always, 

 characterized by a uniform dark or brownish color which is due to inclusions of 

 limonite.* 



Prof. F. H. Knowlton, of the United States Geological Survey, has studied raicrc- 

 scopically both the lignite and silicified woods found in Eastern Arkansas. The 

 results of his work may be found in Vol. II. of the Arkansas Geological Survey 

 Reports for 1889. His studies have developed the interesting fact that the woods 

 belong to both dicotyledonous and coniferous types. This occurrence is the first 

 known dicotyledonous wood found in this country in rocks older than Pleistocene 

 and is the first dicotyledonous form determined by internal structure. If, there- 

 fore, examinations of both lignites and silicified woods are made and it results that 

 the same form or forms are represented in both, a strong reason exis-ts for genetic- 

 ally connecting the silicified woods with the lignites. 



Unfortunately for taxonomic purposes all the forms described by Prof. Knowlton 

 are new, but some otherwise valuable results have been reached. In the first place 



* An especially fine example of this nature was taken from a section in Tertiary 

 sands thirteen miles southeast of the town of Camden on the line of the Camden & 

 Alexandria Railroad. Of the many thousands of quartz crystals which this specimen 

 exhibits not one has been seen which is free from inclusions of limonite. 



