MEMOIRS 
PRESENTED TO THE CALIFORNIA ACADEMY OF SCIENCES. 
VOLUME I. 
Il. Prineiples of the Natural System of Volcanie Rocks. 
By F. Baron RiIcHTHOFEN, Dr. Pur. 
[Presented, May 6th, 1867.] 
| AERO ae Among the features peculiar to modern Geology may be noticed 
a revival of that speculative tendency which prevailed among the cultivators of this 
science at the close of the last century. But while in those early times imagination 
exerted a dominant influence in the framing of hypotheses, and discussions between 
the adherents of different doctrines were conducted with all the bitterness peculiar to 
such struggles, when neither party has a firm basis upon which to found its arguments, 
‘the constant ascendency of the spirit of the inductive method has imparted to those 
theories more recently propounded a more logical and scientific form, while, at the 
same time, the increasing amount of positive knowledge has given to the different 
doctrines a more varied and more definite character, and enlarged the scope of dis- 
senting views. 
This renewed tendency to systematize and theorize, which is especially con- 
spicuous in the records of the last twenty years, must be ascribed, partly, to the vast 
amount of well-established facts gathered during the previous decades, and which 
have since been multiplied and intensified in a constantly increasing ratio, as regards 
depth and distinctness of observation as well as the geographical area over which they 
extend ; partly, and in no less degree, to the rapid progress made by those sciences on 
which geology has to draw for the general laws which are alone capable of affording a 
philosophical guide to speculation on the basis of facts gained by observing and com- 
paring. The advance of the chemical and physical sciences, especially, has had a 
MEM. CAL. ACAD, SCI. VOL, 1. [2r] Jan. 1868, (89) 
