Introduction to Geology. TL 
and schists, or to beds of sand; while the sandstones, losing 
their cohesion, become siliceous clays, or friable sands; the 
sands pass into chert, and the gravel into breccia, or conglo- 
merates. Of this changeful property the red marl, or ‘red 
sandstone formation, is an instance. ‘The lias, the ee 
and the ferruginous sands, afford numerous examples of 
similar tendency. 
It must not be inferred that these deposits, to which we shall 
hereafter more specifically advert, present an absolute uni- 
formity of character in every part of the world, or even that 
‘they can be at all times satisfactorily idewuned at remote 
points. Modifications of those characters by which they are 
best distinguished within the limited area of our island, enor- 
mous expansions in some cases, the introduction or the nbsches 
of certain beds or members in others, increase the difficulty of 
the investigation. At such times the observer has recourse to 
the organic remains, as a frequent means of removing his 
doubts when most other circumstances are equivocal. 
Such is the diversity observable in the composition and 
external features of the secondary formations, that the study 
of this division is infinitely more attractive, and in its results 
more singular than that of the primary order, and its mineral 
products. ‘The knowledge of these substances, which is the 
wtimate object of the mere mineralogist, is to the geologist 
only a subordinate acquisition, and for ms but the alphabet by 
which he endeavours to decipher the chapter of nature which 
he studies. Hence the rarer varieties which, in the estimation 
of the mineralogist, possess the highest interest, will, in the 
eyes of the ecologist, attract the least re egard.” 
There is little doubt that the advances which have been made 
in the developement of the secondary strata have been mainly 
owing to the high interest which their extraneous fossils have 
excited. A powerful stimulus has also been derived from the 
writings of the celebrated French naturalist, Baron Cuvier, 
whose discoveries have thrown a charm over this branch of 
science, and strewed the path of all succeeding geologists with 
flowers. “ This study is rendered interesting,” he observes, 
** by the variety of pr oductions of partial or general revolutions 
which it affords, and by the abundance of the different species 
which alternately offer themselves to view; it neither has that 
dull monotony which attaches to the study of the primitive 
formations, nor does it force us, like the latter, almost neces- 
sarily into hypotheses. ‘The facts with which it is conversant 
are so prominent, so curious, and so obvious, that they may 
suffice to occupy the most ardent imagination; and the con- 
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