of the Gold-fields of California. 335 



portions consist of larger boulders than the upper; but this does 

 not exclude the occasional appearance of large rounded masses 

 of rock among the middle or upper members of the series. 



Water-worn gold is, to a greater or less extent, disseminated 

 throughout the whole mass of these deposits ; not, however, with 

 uniformity, but always with greater abundance near the bottom, 

 and more particularly in direct contact with the bed-rock, which 

 is invariably grooved and worn by the action of water. The 

 materials of which these deep placers are composed are frequently 

 consolidated into a sort of hard concrete by being firmly bound 

 together by crystalline iron pyrites ; and sometimes this cement- 

 ing material partially consists of carbonate of lime and amor- 

 phous silica. In many cases, as at some places in the county of 

 Nevada, the sand, gravel, and boulders are firmly held together 

 by a material consisting almost entirely of silica, in such instances 

 giving rise to an exceedingly hard conglomerate. 



When in the deep placers this consolidation of the materials 

 of which they are composed has taken place, it is known by the 

 name of cement, and often necessitates the employment of large 

 quantities of gunpowder for its removal. 



The silica forming one of the ingredients of the auriferous 

 conglomerates is rarely met with in a crystalline form; but near 

 Kenebeck Hill I found a cavity, resulting from the junction of 

 several pebbles, completely lined with well-defined crystals of 

 quartz. One of these, on being sufficiently reduced in thickness 

 by grinding, did not show under the microscope the usual fluid- 

 cavities of the quartz of the ordinary veins of the country. In 

 some localities the silicified wood is found in the form of large 

 trees, of which the entire mass has become replaced by a siliceous 

 deposit ; whilst in others this transformation has taken place to 

 a limited extent only, and on breaking up a large trunk the cen- 

 tral portion still retains the structure and properties of the 

 original wood. 



Where the cementing material of the conglomerate chiefly 

 consists of pyrites, the enclosed trunks of trees are usually re- 

 placed by that mineral, although, of two pieces of wood lying in 

 close proximity to each other, one may have become silicified, 

 whilst the other is replaced by iron pyrites. On examining 

 under the microscope the leaves which often form thin beds in 

 the sands and clays of the tertiary auriferous river-beds of the 

 gold-regions, they are seen to be covered by small guttate depo- 

 sits of semitranslucent silica; and on burning some leaves found 

 in these drifts in the county of Nevada, I found that, after long 

 digestion in hydrochloric acid, they afforded 25*21 per cent, 

 of a siliceous ash. 



A specimen of silicilied wood (coniferous) from French Corral 



