VEGETABLES 187 



Petrified wood occurs in a number of our strata, as in the sand- 

 stone, the ironstone, the dogger, and especially the aluminous 

 beds. Large portions of trees, some of them with the appearance of 

 roots, have been found in the sandstone; but smaller pieces of wood 

 are much more common. Very often the wood in the sandstone is 

 in the state of charcoal, some of the strata containing an infinite 

 number of fragments of charred wood, of various sizes. Beds of this 

 description may be seen between Cayton mill and Scarborough, and 

 especially among the sandstone strata immediately over the alum 

 rock, both on the shore and in the interior. 



In the aluminous strata, petrified wood is found in abundance, 

 and in a variety of forms. Fig. 6 of Plate III exhibits a small 

 specimen, containing a knot, from whence a branch has issued. The 

 grain of the wood is very discernible, as it usually is in what we find 

 in the alum rock. Sometimes the wood contains calc spar, disposed 

 in veins or cracks, after the form already noticed as occurring in the 

 stalks of plants in the Saltwick ironstone; the wood being rent or 

 cracked both longitudinally and transverselj^ as in charred wood, 

 and the sparry matter filling up all the cracks. Very often, instead 

 of spar, we see pyrites occupying the fissures; and a great proportion 

 of the specimens in the alum shale have more or less of a pyritous 

 crust. In many instances too, siliceous matter is distributed through 

 the veins or cracks, and it is not uncommon to find specimens highly 

 siliceous, or completely agatized. In almost all the siliceous spe- 

 cimens, however, and in a great part of the other varieties, whether 

 calcareous, pyritous, or argillaceous, the wood has no appearance of 

 having been charred, and the stony matter, instead of being distribu- 

 ted in veins or cracks, is diffiised through all the pores and inter- 

 stices of the Avoody fibres, with which, especially in the siliceous 

 petrifactions, it is intimately combined. It is not uncommon to find 

 all or most of the varieties, now mentioned, in the same specimen;. 



>^: 



