PREFACE. 
The original MS. of the 'Principles of Geology' 
was delivered to the publisher at the close of the 
year 1827, when it was proposed that it should 
appear in the course of the year following, in two 
volumes octavo. Since that time many causes have 
concurred to delay the completion of the work, and, 
in some degree, to modify the original plan. In May, 
1828, when the preliminary chapters on the History 
of Geology, and some others which follow them in 
the first volume, were nearly finished, I became 
anxious to visit several parts of the continent, in 
order to acquire more information concerning the 
tertiary formations. Accordingly, I set out in May, 
1828, in company with Mr. Murchison, on a tour 
through France and the north of Italy, where we exa- 
mined together many districts which are particularly 
mentioned in the body of this work. We visited 
Auvergne, Velay, Cantal, and the Vivarais, and 
afterwards the environs of Aix, in Provence, and 
then passed by the Maritime Alps to Savona, thence 
crossing to Piedmont by the Valley of the Bormida. 
At Turin we found Signor Bonelli engaged in the 
arrangement of a large collection of tertiary shells 
