172 CONIFEEALES [CH. 



of former ages : the petrified forests in the desert east of Cairo ^ ; 

 the vast accumulation of Triassic stems, some reaching a length of 

 .200 ft. and 7 — 10 ft. in diameter, over an area of 10 square miles 

 in Arizona County^ ; the succession of Tertiary forests in 2000 ft. 

 of volcanic sediment exposed on the sides of Amethyst Mountain 

 in the Yellowstone Park^ (fig. 712) ; and on a smaller scale the 

 Jurassic trees in the Portland quarries are a few of many striking 

 examples of the wealth of material. 



Classification of fossil Coniferous wood. 



A. Araucarian fitting on the tracheids. 



I. DADOXYLON. (Including Araucarioxylon of Authors.) 



Bordered pits on the radial walls of the tracheids, if uniseriate 

 flattened above and below,' when in two or more rows alternate 

 and polygonal ; separate and circular pits, though rare, may occur. 

 Rims of Sanio usually absent though their occasional occurrence 

 on the secondary tracheids in the cone-axis of recent species of 

 the Araucarineae shows that they are not entirely foreign to wood 

 of the Araucarian type. 



Xylem-parenchyma absent or rare and may be represented by 

 resiniferous tracheids. Medullary rays uniseriate, rarely double ; 

 horizontal and tangential walls smooth; there may be 1 — 15 

 small pits in the field, though whether they actually belong to the 

 walls of the ray cells or to the adjacent tracheids has not been 

 definitely determined in fossil species. The pits are simple or 

 bordered, circular or elliptical. 



Resin-canals are absent both from normal and wounded wood. 

 Palaeozoic to Recent. 



B. Pitting on the radial walls of the tracheids of the common 

 Coniferous type ; the pits are separate and circular and, if in two 

 or more rows, opposite. Contiguous and more or less flattened pits 

 occur spasmodically on the tracheids of the wood of ike genera included 

 in this section. Well-developed rims of Sanio usually occur on the 

 tracheids. 



1 Unger {59). 



^ A good example of an Arizona tree-trank is exhibited in the Plant-Gallery of 

 the British Museum. See Ward (00^). 



» Knowlton (99); Holmes (78); Seward (ll') p. 60, fig. 6. 



