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MEMOIRS 



PRESENTED TO THE CALIFORNIA ACADEMY OF SCIENCES. 



VOLUME I. 



II. Principles of the Natural System of Volcanic Rocks. 

 By F. Baron Riciithofen, dr. phh,. 



[Presented, May 6th, 1867.] 



INTRODUCTORY. 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. i. [2e] Jan. 1868. (39) 



