42 IOWA ACADEMY OF SCIENCES. 



be more distinctly stated that either fossil wood or leaves have been found at every 

 elevation, from the lowest to the hig:hest, where gravels occur. iLven as high a& 

 Silver Mountain City, at 7,000 feet of elevation, large masses of fossil wood are 

 found in the volcanic deposits; and in Plumas county the same occurrence has been 

 noted on several of the highest mountains in the region, as Penman's Peak and 



Clermont, peaks from 7,000 to 8,000 feet high Fragments and often 



large masses of wood are found, both in the gravels and the associated clayey and 

 tufaceous beds. In the gravel they frequently bear the marks of transportation 

 from a distance, as would be expected." 



In the California Tertiary the most completely silicified and best preserved 

 specimens of wood occur in connection with deposits of a volcanic character, some- 

 times a rhyolitic ash.* It is suggested by Whitney that these relationships have 

 something to do with the process of silicification. For that region Whitney 

 believes that not only were the woods silicified after their imbedding in white 

 pulverulent volcanic ash but "the lava itself exhibits signs of having been acted on 

 by silicifying agents after its deposition." That the greater part of the series of 

 beds included in the gravel formation has been thoroughly permeated with waters 

 holding silica in solution and that chemical changes induced thereby are sutticient 

 to explain the phenomena appears quite probable. The relations which the phe- 

 nomena sustain to the facts of volcanism so abundant in that region are set forth 

 and the conclusion is drawn that that relation explains silicification in these 

 woods. In California it becomes a subordinate problem under volcanism. 



The chemical processes which obtained in the case of the Arkansas gravels were- 

 not co-ordinate with those in California, for there is no evidence of volcanism or 

 any similar phenomena associated with their silicification. The silica in the east 

 em locality must be sought in the accompanying sand beds and was probably 

 brought into solution by the action upon it of organic acids. 



The study of the Arkansas Tertiary silicified woods appears to justify the follow- 

 ing conclusions: 



1. The silicified woods of Eastern Arkansas are all of Tertiary age. 



2. They are derived from the beds of Eocene clays that underlie the sands 

 and gravels in which they commonly, occar. 



3. They are silicified lignite; the process of silicification has occurred either 

 while they were still in the clays or most often after they were removed and buried 

 in the sands and gravels. 



4. They possess as yet no taxonomic value in determining the relative ages ot 

 the members of the Tertiary series. 



ADDITIONAL NOTE ON SILICIFIED WOOD IN IOWA. 



Nearly all who have had occasion to make any extended study of the Pleistocene 

 strata or deposits in Iowa have found, somewhat farely it is true, specimens of 

 silicified wood which occur under varying conditions. Most of those which 

 the writer has seen have been found in rearranged Pleistocene strata and bear 

 evidence of having been rather roughly handled since silicification. The generic 

 position of most of these examples is uncertain since there have been no careful 

 microscopical examinations, save in a single instance, of any of these specimens. 

 Professor F. H. Knowlton, of the United States Geological Survey, has studied 

 very carefullyt a single example of these woods, basing his investigation upon a 



♦Op. cit., pp. 32;-329. 



+ Proceedings United States National Museum, Volume 11, 1888, p. 5-6. 



