1892 - 93 .] H. B. Stocks on the Lower Goal Measures. 75 
position still proceeding would lead to the formation of carbonate of 
lime upon the outside of the parts, and thus the outer markings, 
being covered, are also protected ; subsequently the unpetrified 
remainder of the plant decayed away, leaving broken ends upon 
which layer upon layer of mineral matter would form, each layer 
tending to bring the mass nearer to a spherical shape, until finally 
all decomposition in the wood ceased, and the concretion had 
attained its present size. There is no evidence that these nodules 
have grown in size since the time the coal was formed. 
The reason for the mineral matter aggregating in the form of 
spheroids lies in the fact that when carbonate of lime is deposited 
slowly from a solution containing much organic matter in solution, 
it is prevented from assuming the crystalline character, but owing to 
the physical conditions, it assumes a more or less spherical appear- 
ance, as we see in the present case. As an example of this I quote 
the experiment of Mr George Eainey, mentioned in Hogg, On the 
Microscope, ed. 1854, who found that when a solution of calcium 
chloride and a solution of sodium carbonate, each in gum water of 
the same specific gravity, were carefully placed in a bottle so that 
mixture did not take place immediately, small spherules of carbonate 
of lime began to form and grow by the gradual diffusion of the 
liquids. 
The process of petrifaction and the formation of these concretions 
is a difiScult subject to deal with, seeing that our knowledge is as yet 
only limited. I have not been able to find any recorded theory apart 
from accretion to account for their formation ; very little has up to 
the present been written upon petrifaction, although it is a subject 
of great interest. We have in these nodules specimens of plants 
showing perfectly every detail, not only external, but internal, and 
the question is, — How were they preserved in such a beautiful 
manner for ages ? In the opinion I have expressed will be found 
an answer; and, moreover, I have a number of specimens of plants 
under treatment with a view to test the truth of the theory. 
