PREFACE. XV 



Eeaders will perhaps notice the absence of illustrations of magnified 

 thin sections in this volume. After having presented in a former report 

 illustrations of this kind which are generally acknowledged to be unsur- 

 passed b}' any yet published, I have come to the conclusion that the lessons 

 which they teach do not repay their cost in time and mono}-. 



1 have thought it best to make each chapter iii this volume as far as 

 possible independent of the rest. In doing so I am sure that I meet the 

 wishes of many readers who will care to consult only certain portions of 

 the book. This plan, however, involves some repetition, which may prove 

 wearisome to continuous readers. I crave their indulgence in this res2:)ect 

 for the sake of the larger class. Personally, I should prefer never to re- 

 state a fact or an opinion. 



After the manuscript of this report was substantially completed I was 

 authorized to visit the great Almaden iTiine in Spain and the Tuscan deposits. 

 Such a visit was almost essential to the purposes of the investigation ; for the 

 results which I had reached from study of the American deposits differed in 

 important respects from the conclusions of some geologists respecting the 

 great Spanish deposit If they were right, it became necessary to warn 

 American miners that cinnabar might be looked for under very different 

 conditions from those described in this volume. If the greatest quicksilver 

 deposit of the world proved similar in its mode of occurrence to those of 

 California, the conclusions drawn from the latter would gain greatly in 

 strength. I had the satisfaction of finding that the deposit of Almaden 

 showed an association with eruptive phenomena, a structure, and a mineral 

 association similar to those which are typical of the Pacific slope. Such 

 statements as that the Almaden ore bodies are not veins, that the cinnabar 

 is free from other sulphides, that it is accompanied by no gangue minerals, 

 that it was deposited with the inclosing rocks, that it is deposited by sub- 

 stitution for sandstone, and that there is no evidence of a connection between 

 the deposition and eruptive phenomena — these alk^gations are, in my judg- 

 ment, erroneous. The Tuscan deposits, too, I found similar to some in Cali- 

 fornia. Only a few notes concerning my studies of these mines are included 

 in this volume. I e.xuect to write more fully of them hereafter. 



July, 1887. 



