380 Walton.—On a New Method of investigating Fossil 
fractured surfaces of blocks of petrified wood. Sometimes the wood has 
been rendered so transparent and homogeneous in the processes of petrifica¬ 
tion that no structure can be made out in thin sections of the material. 
This is not infrequent in silicified wood. In other specimens the wood is 
quite opaque, when, for example, its structure is preserved in pyrites. In 
both these types of fossil, examination of a radial fracture will often afford 
valuable data concerning the structure of the secondary wood : such features 
as the medullary rays and the pitting on the tracheides show up clearly. 
In a specimen of Rhexoxylon africanum , Bancroft, a fossil wood from 
South Africa of Triassic Age, the pores on the bordered pits showed up 
very definitely. Similar perfection of detail was observed in a portion of 
a jasper tree (Triassic) from Arizona in America. In both these specimens 
the preservation is such that practically no structure is visible in thin 
sections on examination with transmitted light. A piece of pyritized wood 
from the Lower Estuarine beds of the Jurassic of the Yorkshire coast 
showed the features of the pitting very distinctly. 
In a few exceptional cases the fossil plant incrustation may become 
detached from the rock and then both surfaces can be examined. Hamshaw 
Thomas 1 described specimens of Thinnfeldia fronds from the Middle 
Estuarine of Yorkshire that can be stripped with ease from the rock. 
Sometimes such fossils are transparent and then more information is 
available. Miss Wills 2 describes cuticles of Carboniferous plants found in 
clayey shales in the Upper and Middle Coal Measures which could be 
removed from the shale by soaking. The fact that coherent samples of the 
specimens can be obtained is largely due to the excellent state of preser¬ 
vation of the cuticles of the upper and lower surfaces of the leaves. 
A. G. Nathorst 3 described Tertiary coniferous twigs from Ellesmere Land, 
with the leaves attached, which could be similarly removed from the rock. 
There are also, of course, intermediate examples in which, by exercising 
considerable care, small fragments of plants may be detached ; but oppor¬ 
tunities such as these are rare, and it is not often that we are able to 
examine specimens which, as Hamshaw Thomas says, 4 may be regarded 
as true examples from the £ Herbarium Diluvianum ’. 
With the realization that a considerable amount of the original plant 
substance was still preserved in fossil plants, and that the cuticle still 
exists in an almost unchanged condition, a great advance in the study of 
these plant impressions was made possible. Schulze 5 in 1855 introduced 
a method of isolating portions of the cuticles of such fossils by chemical 
treatment, and Nathorst employed the same method in many of his 
researches. In this country Hamshaw Thomas and several other palaeo- 
1 Thomas, H. Hamshaw (1913). 
8 Nathorst, A. G. (1915). 
5 Schulze, F. (1855). 
2 Wills, Lucy (1914). 
4 Thomas, H. Hamshaw (1915). 
