Chap. I.] 
LAURENTIAN— THE OLDEST SYSTEM. 
9 
Ural Mountains, and Scandinavia, the sequence was thus followed out, 
from the most ancient fossil-bearing strata of those regions to the latest 
stages in the geological series. 
The leading object, therefore, of the present work is not, I repeat, to 
bring out the ' Silurian System' in a mere abridgment of its original form, 
but to concisely describe the Silurian system of rocks, such as that system 
became in subsequent years, before it obtained the highest distinction which 
the Eoyal Society bestows*, and what it long since proved to be, with the 
additions made to it by other geologists at home and abroad. 
In the present volume attention is chiefly restricted to the consideration 
of the earliest great eras of life. The plan, therefore, pursued will be, so 
far, similar to that which was adopted in the large work on Eussia ; and 
these leaves of geological history will be written from the first traces we 
have obtained of organic remains, — a plan which, for want of knowledge, 
was impracticable when the ' Silurian System ' was issued in 1838. 
After 1854, when the first edition of the present work was published, and 
even after many pages of the second edition were printed, a most important 
addition was made to our acquaintance with the lowest or fundamental 
rocks of America and Europe, to which it is necessary at once to allude. 
The Laurentian or Oldest known Stratified Bocks. — In the last edition of 
this work it was shown, and even represented in the coloured frontispiece, 
which is now repeated, that the classification previously adopted by British 
geologists, and which assumed that the Cambrian rocks, or their altered 
equivalents, were the oldest deposits in our islands, had been set aside by 
evidence proving the existence of stratified rocks beneath all those to which 
the terms of Cambrian and Silurian had been applied. 
My scientific cotemporaries are aware that I was the first to announce 
the existence of Laurentian rocks in Britain, as proved by a true order of 
their infraposition to Cambrian and Lower Silurian rocks, which I ob- 
served on the north-western coast of Scotland. This view was communi- 
cated to the Geological Society of London, and accompanied by a new geo- 
logical map of the Highlands, on which the Laurentian gneiss was clearly 
separated from the younger gneissic and crystalline rocks occupying so large 
a portion of that region (see Quart. Journ. Geol. Soc. Lond. 1848-49). For, 
although MacCulloch and others had shown that the gneiss forming the 
lower headlands of the west coast of Sutherland was unconformably sur- 
mounted by red sandstone and conglomerate, they did not possess at that 
time the means of testing the relative ages of the various rock-formations. 
In those days the Silurian classification was unknown, no organic remains 
had been discovered in the limestones of Sutherland ; and Professor Sedg- 
wick and myself had simply observed, in the year 1827, that the lime- 
stones which have since proved by their imbedded fossils to be of Lower 
Silurian age passed conformably under micaceous schists and quasi-gneissic 
* The Copley Medal (1849). 
