IOWA ACADEMY OF SCIENCES. 41 



Ity the waters which deposited the sands above the clays.* As ordinarily under- 

 Btood the process is purely a chemical one and perhaps very slow. It consists in 

 the replacement, particle by particle, of the carbon of the li<;nite by silicic acid, or 

 silicon dioxide. It is by no means essential that theorfjanic matter be unchanged 

 when the process begins. If the belief that this wood represents what was once 

 lignite be a correct one, then the process of silicification can occur in the case of 

 •organic matter which has already undergone a partial change. 



Where found in clays in a silicified condition, it has probably resulted from the 

 same processes that are seen to obtain in the highly siliceous sands or gravels 

 which overlie them. Though the impervious nature of most clays renders the 

 •percolation of silica-charged waters a matter of great ditticulty such percolation 

 •certainly occurs in them. The silicified masses of wood are often far too large to 

 •have been removed from the clays and deposited in the overlying gravels by an 

 •ordinary wave or current action, for they sometimes weigh tons. In the form of 

 iignite the same masses could have been transported by currents, but since very 

 large pieces have been rarely, if ever, found far from lignite deposits, even that 

 proposition has very little weight. 



The vertical distribution of the silicified woods of the Arkansas Tertiary is 

 limited by the line of contact between the sands and clays which constitute the 

 Arkansas series. Below this line the silicified wood never occur.s with the single 

 exception above, f so far as observations have yet extended. Above it no lignites 

 have ever been found. The vertical range is therefore limited by the thickness of 

 the sand and gravel bed which is commonly, in Arkansas, between fifty and eighty 

 feet. 



There is a marked difference in the vertical range of (his fossil in the Tertiary of 

 Arkansas and the Tertiary of California. In the latter State the vertical range is 

 often many hundreds, even several thousands, of feet. Whitney says 4 "It will 

 be proper to add some of the most important facts gathered during the investi- 

 gation of the gravel deposits in regard to the mode of occurrence of the fossil plants 

 of the Pliocene epoch. The vertical range of these has been alluded to, and it may 



*Dr. K. A. F. Penrose, Jr. (op tit., pp. 24, 26, 50, et seq ). has placed on record the 

 numerous occurrences of silicified wood in the Tertiary of Te.xas; he finds it in both 

 sands and clays. In his description of the Sabine Kiver beds he says: "Silicified wood 

 is of very frequent occurrence in tlicse strata; sometimes occurring as small frag- 

 ments; and at other times as large trunks of trees. On tlie Brazos River, in the^ 

 northern part of Milam county, was seen a trunk one and a half feet in diameter, pro- 

 truding from a clay bed. Ten feet of it were exposed, while the rest was imbedded in 

 the clay. In many places such fragments are collected in great quantities, but it is 

 especially plentiful in the lower part of the Fayette beds. It is generally dark brown 

 or black inside, and weathers gray or butf color on the outside. Sometimes it occurs 

 partly lignitized and partly silicified. It frecjontly shows shrinkage cracks which are 

 filled with quartz or clialcedony. and are often lined with quartz crystals." 



In this case stratification was but partial or was still in progress, and since there is 

 exposed in the face of the bluff a log which was partially lignitized and partly 

 silicified it proves all but conclusively that, even in the Texan Tertiarios. the lignitic 

 precedes the siliceous condition of these woods. 



tin this case the stumps are still standing, the roots, also silicified, ramifying in all 

 directions in Eocene blue clays. Less than one hundred feet east, however, tlie line 

 of contact between the sand beds and tlie clays was disclosed in a vertical cut in a 

 liillside. This line was at or near the elevation of the stumps. It was clear that, if 

 the stumps did not actually project into the overlying sands, they were but a short 

 distance below and under conditions to favor silicification from waters percolating 

 through the clay to them. 



•t Auriferous Gravels of the Sierra Nevada, pp. 2:ii. 2;if.. See also American Journal 

 of Science. II, Vol. XLI, p. 359. 18t56. 



