108 Geological Society of London. 
Dr. Fitton has annexed to his excellent monograph on the strata 
below the chalk, just published in our Transactions, (2nd Series, vol. 
iv. part 2.) you will see that a considerable number of shells pass 
from the upper oolitic groups into the green-sand. We are not to 
conclude from these facts that certain sets of fossils may not serve as 
good chronological tests of geological periods, but we must be cau- 
tious not to attach too much importance to particular species, some 
of which may have a wider, others a more limited vertical range. 
The phenomena alluded to are strictly analogous to those with which 
we are familiar in the more modern deposits where different tertiary 
formations contain some peculiar Testacea, together with others 
common to older or newer groups, or where shells of species now 
living in the sea are associated with others that are extinct. 
An assemblage of fossil shells has been presented to our museum 
by Mr. J. Leigh and Mr. J. W. Binney, found at Collyhurst near 
Manchester, in red and variegated marls, which were referred by 
them at first to the upper division of the new red sandstone group ; 
but Professors Sedgwick and Phillips consider them to be a red and 
variegated deposit, belonging to the magnesian limestone series. As 
these fossils are new and characteristic of a particular subdivision of 
the beds between the lias and coal, it is to be hoped that they will 
soon be described and figured. 
The petrifaction of wood, and more especially its silicification still 
continues to present obscure problems to the botanist and chemist. 
The first step towards their solution will probably be made by care- 
fully examining vegetables in different stages of petrifaction, and . 
with this view Mr. Stokes has procured several specimens of wood, 
partly mineralized and partly not. Among these is a piece found in 
an ancient Roman aqueduct in Westphalia, in which some portions 
are converted into spindle-shaped bodies consisting of carbonate of 
lime: while the rest of the wood remains in a comparatively un- 
changed state. The same author has pointed out cases both of sili-- 
eeous and calcareous fossils, where the lapidifying process must have 
eommenced at a number of separate points, so as to produce spheri- 
eal or fusiform petrifactions, independent of each other, in which the 
woody structure is apparent, while in the intervening spaces the 
wood has decayed, having after removal been replaced by mineral 
matter. In some petrifactions, the most perishable, in others the 
most durable portions of plants are preserved, variations which 
doubtless depend on the time when the mineral matter was supplied. 
