a piece of rock, composed, we shall say, of sandstone, which 
has just been broken from the solid bed in the side of a hill. 
In that piece of rock, and as it lay in the mass of the 
mountain, you see the form of a shell. The words which 
express the thought of that fact form a part of that which is 
fundamental in geology. Apart from this kind of thought 
there is nothing real in the science. 
In that which is called a fact of this character, you have 
three things; first, the material rock with its shell -form ; 
then the thought representative of that object in the mind; 
and third, the words which express that thought. The piece 
of rock is the same to all who see it ; the thought repre- 
senting it in one mind is probably, so far, unlike the thought 
of it in every other ; and the words expressive of such thought 
are both varied and changeable. Yet, from the nature of the 
rocky fact itself, there is at least a possibility of such repeated 
observation as issues in the all but perfect agreement of 
informed minds, as to the thing itself. It is the expression of 
thought regarding such facts, about which the truly scientific 
mind is ever most careful. 
But to proceed to another example. You are on the sea- 
shore ; and observing a portion of the sand which the tide has 
left exposed, you see that true shells, as they have been left by 
the molluscs that dwelt in them, are imbedded in that sand 
exactly as the form you have seen is imbedded in the rock. 
As yet we assume that you do not reason on the relations of 
those objects — you only observe them as they lie. Your 
thoughts represent little more than that which has reached you 
through your senses, sufficiently cogitated to present the 
objects to your mind. We shail suppose that you go on ob- 
serving objects of this character, you are treasuring that kind 
of thought, out of which all geological science must be 
formed. 
But there is, as we have said, a second and very different 
description of geological thought. You bring together the 
form of a shell which you have observed in the rock, and a 
real shell which you observed in the sand; comparing them, 
you perceive that, in many respects, they are not alike. They 
are indeed similar, but also strikingly dissimilar, and you 
begin to reason or to infer , that is, to form certain thoughts 
which represent relations of objects rather than the objects, 
themselves; You then leave the thoughts representative, of 
the mere facts for totally different thoughts, and enter a region 
in which difficulties and dangers greatly increase. It is then 
that you begin to realize what Steno, one of the ablest of 
geologists, wrote about two centuries ago. He says, addressing 
