80 THE PRESERVATION OF PLANTS AS FOSSILS. [CH. 



Conwentz^ have described silicified trees more than fifty feet in 

 length from a locality in California where several large forest 

 trees of Tertiary age have been preserved in volcanic strata. 

 In South Africa on the Drakenberg hills there occur numerous 

 silicified trunks, occasionally erect and often lying on the 

 ground, probably of Triassic age^ In some instances the 

 specimens measure several feet in length and diameter. Some 

 of the coniferous stems seen in Portland, and occasionally met 

 with reared up against a house side, illustrate the silicification of 

 plant structure on a large scale. These are of Upper Jurassic 

 (Purbeck) age. From Grand'Croix ia France a silicified stem 

 of Gordaites of Palaeozoic age has been recorded with a length 

 of twenty meters. The preservation of plants by siliceous infil- 

 trations has long been known. One of the earliest descriptions 

 of this form of petrifaction in the British Isles is that of stems 

 found in Lough Neagh, Ireland. In his lectures on Natural 

 Philosophy, published at Dublin in 1751, Barton gives several 

 figures of Irish silicified wood, and records the following 

 occurrence in illustration of the peculiar properties erroneously 

 attributed to the waters of Lough Neagh. Describing a certain 

 specimen (No. XXVl), he writes : — 



" This is a whetstone, which as Mr Anthony Shane, apothecary, who 

 was born very near the lake, and is now alive, relates, he made by putting 

 a piece of holly in the water of the lake near his father's house, and fixing 

 it so as to withstand the motion of the water, and marking the place so as 

 to distinguish it, he went to Scotland to pursue his studies, and seven 

 years after took up a stone instead of holly, the metamorphosis having 

 been made in that time. This account he gave under his handwriting. 

 The shore thereabouts is altogether loose sand, and two rivers discharge 

 themselves into the lake very near that place ^." 



The well-known petrified trees from the neighbourhood of 

 Lough Neagh are probably of Pliocene age, but their exact 

 source has been a matter of dispute*. 



In 1836 Stokes described certain stems in which the tissues 

 had been partially mineralised. In describing a specimen of 



1 Conweutz (78). 



'^ A large piece from one of these South African trees is in the Fossil-plant 

 Gallery of the British Museum. 



3 Barton (1751) p. 58. ^ Gardner (84) p. 314. 



