268 



SEDIMENT-FILLED VEINS 



tioned above. In the Mississippi valley, from Wisconsin to 

 Arkansas, are found great quantities of galena, the sulphide of 

 lead (PbS), deposited in cavities of limestone ; the cavities follow 

 the lines of joint and are largest along the master joints. Associ- 

 ated with the lead frequently occur ores of zinc, pyrite, marcasite, 

 and other minerals. How these remarkable accumulations were 

 formed is altogether doubtful. 



. \,. 



FlG. 116. — Dikes of sandstone in shales. Northern California. (U. S. G. S.) 



Sediment-filled Veins. — Vertical fissures are sometimes filled 

 up by sediment washed in from above, but more remarkable are 

 the instances where the fissure evidently did not communicate 

 with the surface and yet was filled with sediment different from 

 the walls. In Fig. 116 is seen an example from northern Cali- 

 fornia : the fissures which traverse the shale have been filled with 



