4 INTRODUCTORY. 



ascend from the examination of the nature of these rocks to that of their mutual 

 relations, to investigate these from as many points of view as we can discover, in re- 

 gard to physical and chemical properties, mode of occurrence and age, as well as in 

 regard to geographical distribution : that is, to try to establish the natural system of 

 eruptive rocks. The results so obtained may then, in accordance with what we just 

 remarked in a general way, be applicable to reasoning on remoter questions, of 

 which we can only attempt to find the most probable solution. They regard chiefly 

 the causes of those relations, the mode of origin of the eruptive rocks, and the 

 processes connected with their ejection. The intricate nature of the subject, and 

 the fact that the present changes on and below the face of the globe, as well as 

 the events of the past, are often but dimly and imperfectly perceptible to our ob- 

 servation, demand that we should concentrate our endeavors in exploring first the 

 laws of that which is definite and constant within the infinite range of phenomena, and 

 await further experience to arrive at an explanation of those isolated facts which form 

 apparent exceptions to the order of things. 



It is with these views that the following pages were written. They extend 

 chiefly over the comparatively limited, and yet very extensive class of "volcanic rocks," 

 and are offered as a mere elementary attempt, which is necessarily very imperfect. 

 The application to exact reasoning of the numerous observations which have been made 

 on the subject of volcanic rocks in different countries, is nearly prevented by the ex- 

 traordinary discrepancy existing in regard to the mode in which the names of rocks 

 are used by different authors. The first condition of a uniform and harmonious mode 

 of observation on volcanic rocks, and the phenomena connected with them, is the ap- 

 plication of a uniform system of nomenclature. 



In concluding these preliminary remarks, I dare express the hope that some in- 

 dulgence may be had with the imperfections of this essay, if it is taken into considera- 

 tion that it was written on the Pacific coast, where chemical laboratories are unknown, 

 libraries scarce, and little opportunity is afforded of becoming acquainted with the 

 current geological literature. I fulfill a deep-felt duty if I tender at this place my 

 sincere thanks to Professor J. D. Whitney, not only for allowing me the use of his 

 library and revising the manuscript of this essay, but also for the interest which he has 

 constantly taken in my pursuits, and for what I owe to his personal intercourse, especially 

 in a country where scientific communication is so extremely limited. The influence 

 of this intercourse will, long after this, be kept in grateful memory by all those who are 

 taking personally a part in the development of the California Academy, the members 

 of which kindly allowed this paper to be published in their Memoirs. 



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