188 ORGANIC REMAINS. 



some parts of a petrified tree, or large piece of wood, being calcareous, 

 others siliceous, others argillaceous, and others pyritous ; or, instead 

 of occupying different parts of the specimen, the petrifying substances 

 may be found blended together. 



In many instances, we find portions of the wood not petrified, 

 but preserved ; being in a soft friable state, like common decayed 

 wood, and retaining its inflammable quality. Such preserved por- 

 tions are found in almost all the varieties above mentioned, not 

 excepting the hardest siliceous specimens ; one part of the same 

 piece of wood being petrified, while another is merely preserved. 

 Sometimes we also meet with preserved pieces in a detached form, 

 imbedded in the alum shale by themselves. Instances also occur, 

 in which only the fibrous part of the wood has been petrified, the 

 sap vessels and pores being left open. In some cases too, especially 

 where the trees are of a considerable size, the bark is found as a 

 crust, enveloping the ligneous matter, as when the tree was growing; 

 but in most specimens, no vestiges of bai'k are to be seen. We do 

 not often meet with trees of a large size, most of the wood being 

 found in fragments, and much of it in a bruised or compressed state : 

 yet round trunks of trees, twelve feet long and upwards, and a foot 

 or more in diameter, have occasionally been discovered at Whitby ; 

 and at Sandsend, RockclifF, and other places. The colour is generally 

 a blackish brown or gi'ey, but it is often yellowish, or light grey ; 

 varying according to the petrifying substance. A transverse section 

 of one of the largest trees is generally very interesting, displaying 

 the concentric zones, or annual growths, together with the veins of 

 pyrites and other substances. 



We have already hinted, that jet is only a variety of petrified 

 wood. That this is the fact, must be obvious to every naturalist 

 who examines the state in which it appears on our coasts. Some- 

 times we find, in the same petrified specimen, one part siliceous, 



J 



