S I L U EI A. 
CHAPTER I. 
INTRODUCTION. 
A GLANCE AT THE PROBABLE EARLIEST CONDITION OF THE EARTH. — STRATIFIED CRYSTALLINE 
ROCKS RESULTING FROM CHANGES OF SEDIMENTARY DEPOSITS. THE SILURIAN SYSTEM 
ESTABLISHED AND EXTENDED. THE LAURENTIAN THE BASE OF PALEOZOIC ROCKS IN BRI- 
TAIN AND ELSEWHERE. — THE EOZOON. — GENERAL PALAEOZOIC SUCCESSION. 
The earliest condition of the earth is necessarily the least susceptible of 
investigation. The favourite hypothesis concerning the primary state of 
the planet, founded on astronomical and physical analogies, is, that it as- 
sumed the form of an oblate spheroid from rotation on its axis when in a 
fluid state. Reasoning upon this idea, and looking to the structure of 
those rocks which either lie at great depths or have been extruded from 
beneath, the geologist has inferred that the crystalline masses, including 
granites, which often protrude from below all other rocks, constituting 
possibly their existing substratum, were at one time in a molten state. 
The theory of an internal heat, at first sufficiently intense to maintain the 
whole terrestrial mass in a state of fusion, but subsequently so far dissi- 
pated by radiation into space as to allow the superficial portion to become 
solid, has been adopted by the greater number of philosophers who have 
grappled with the difficult problem of the primal state of our planet. Most 
of them likewise have believed that all the great outbursts of igneous matter, 
by which the crust has been penetrated and its surface in great measure di- 
versified, were merely outward signs of the continued internal activity of the 
primordial heat, now much repressed by the accumulations of ages, and of 
which our present volcanos are feeble indications. If, then, the mathe- 
matician has correctly explained the causes of the shape of the globe, the 
geologist confirms his views when, examining into the nature of its oldest 
massive crystalline rocks, he sees in them clear proofs of the effects of 
great heat and pressure. The breaking up of the original crust of the 
