410 
The Hon. Mr. Strangways on the 
between those two beds, the lower portion partaking of the nature 
of the clay, the upper of that of the limestone. A minute 
description will therefore be necessary, as it is only by the occasional 
presence of some one or other of its members, that it is to be traced 
throughout a very considerable tract of country ; and as some of 
these members in some parts of the district increase greatly in 
thickness, to the exclusion of those which accompany them in 
others, it is possible that it may exist in other parts of the north of 
Europe, where it has not yet been noticed. It is fortunate, that in 
a part of Europe, so easy of access as the neighbourhood of 
Petersburg, we find some spots where all its varieties may be 
examined in the same natural section. 
I shall therefore give the series of beds which compose this 
stratum, as they occur on the left bank of the river Ishora, exposed 
to view in a precipitous cliff partly overgrown with brushwood, 
but sufficiently accessible to allow of a minute examination of the 
rock, the variations of its character being in this spot more striking 
than in any other I could select. The banks of the Tosna, where 
they may also be seen to great advantage, are less accessible, whilst 
the thickness of the beds is so considerably increased, as not to 
allow of the whole series being seen in one vertical section ; 
a circumstance which adds greatly to the value of those on the 
Ishora, in a practical point of view, as there can exist no doubt 
with regard to their relative position. Each bed is likewise of 
sufficient thickness to present a decided character, and the presence 
of both limestone and blue clay, one at the top, and the other at 
the foot of the cliff, determines at once the place of any of the beds 
wherever it may be met with. 
The spot which I wish to point out for examination is a cliff 
about sixty or seventy feet in height, if not more, on the left bank 
