of the Gold- fields of California. 331 



cavities apparently containing clay, some larger ones in which 

 the clay appears coloured by oxide of iron, and a great number 

 of the markings before referred to, and which are believed to be 

 due to crystallization. 



The most remarkable circumstance in connexion with this vein 

 is the occurrence within it of large quantities of hydrated silica 

 of a bluish-grey colour : this has a resinous lustre, and possesses 

 a distinct conchoidal fracture, but exhibits well-defined lines 

 showing layers of deposition parallel with the other bands of 

 which the lode is made up. It is found in the form of a series 

 of lenticular deposits, often several fathoms in length, interla- 

 minated with the ordinary crystalline quartz, from which it is 

 separated by a thin heading of unctuous clay. This substance, 

 which has the appearance of semi-opal, is frequently 6 or 8 inches 

 in width in its thickest part, and gradually tapers off in all direc- 

 tions in such a way as to assume a lens-like form. This 

 hydrated silica sometimes encloses iron pyrites, and contains a 

 sufficient amount of the precious metal to render its treatment 

 commercially advantageous. 



Examination of Semi-opal from the North- Star Mine. — A spe- 

 cimen of this substance submitted to analysis had a specific 

 gravity of 2*01, and afforded the following results : — 



Water 8*08 



Silica 86-69 



Alumina 1*92 



Protoxide of iron . . . 0*99 



Lime 1*44 



Magnesia ...... 023 



Potash 0-26 



Soda . 0-05 



99-66 

 This substance, at a temperature of 212° F., lost water slowly 

 during several days, and, placed under a bell-glass over fused 

 chloride of calcium, continued to lose weight after 300 hours, 

 when the loss amounted to 5*11 per cent. Under these circum- 

 stances no attempt was made to determine separately the water 

 lost at 212° P. ; but the amount of hygrometric moisture present 

 must have been exceedingly small, as this specimen was taken at 

 the same time and kept in the same cabinet with the crystallized 

 variety, which lost only 0-07 per cent, of moisture at 212° P. 



Examined by polarized light this mineral does not show any 

 evidence of crystallization ; but when a high power is employed, it 

 appears full of small elliptical cavities in which no vacuities were 

 observed. 



Examination of undecomposed Auriferous Slate from Lincoln. 

 — A specimen of the undecomposed auriferous slate from the 



