of the Gold-fields of California. 333 



order to obtain sections affording good examples, even of smal 

 size, it is necessary to select such bands as may be more than 

 ordinarily crystalline, or to operate on thin fragments of crystals 

 sometimes found lining the interior of drusy cavities. 



In the more opaque and generally most auriferous portions of 

 veins, the cavities are numerous but exceedingly small, and are 

 often so opaque (apparently rendered so by being internally 

 coated by a lining of clay) that no vacuities can be distinguished. 

 Some of the bubbles, contained in cavities requiring an arrange- 

 ment magnifying a thousand linear to be distinctly seen, were 

 observed to move freely about. 



Out of more than sixty sections of veinstone examined, only 

 some six or eight were found to contain fluid-cavities of sufficient 

 size to admit of any attempt at accurate measurement ; but in 

 all cases there appeared to be considerable differences in the 

 relative dimensions of the vacuities and the enclosing cavities, 

 and I consequently ascertained by direct experiment the tempe- 

 ratures at which they severally became filled. For this purpose 

 a small bath, made of sheet brass with a piece of microscopic glass 

 fitted into each side, was employed. The section to be examined 

 was clipped on the inside of one of the small panes of thin glass, 

 and the bath rilled with spermaceti into which the bulb of a ther- 

 mometer was inserted. By attaching this apparatus to the stand 

 of a microscope, of which the body was fixed horizontally, heat- 

 ing the bath, and throwing the light of an oil-lamp on the object 

 by means of a condenser, it was easy to study the appearance of 

 the different cavities under various conditions of temperature. 



The magnifying-power employed was in this case about 275 

 linear, and consequently only such sections as contained tolerably 

 large cavities could be satisfactorily examined. In every instance, 

 however, they were found to require very different degrees of 

 temperature to become full, since in the same specimens some 

 of the vacuities disappeared at 180° F., others filled at tempera- 

 tures slightly above that of boiling water, whilst many, though 

 much reduced in size, remained perfectly visible at 365° F.* 



Alluvial Deposits. 



Although a very large amount of the gold annually obtained 

 was no doubt originally derived from auriferous veins, not more 

 than about one-third of the precious metal now collected is pro- 

 cured directly from that source. The larger proportion of the 

 gold brought into the market is derived from alluvial diggings, 



* As an illustration of this fact, the results obtained by heating the sec- 

 tion of a crystal of quartz from the great Mariposa vein may be given. Six 

 several vacuities disappeared at the following temperatures, 250°, 260°, 

 280°, 290°, 310°, and 320° F. Others were not nearly full at 362° F. 



