Phenomena with the Origin of Mineral Veins. 403 



mity of veins, at their inferior extremity, to igneous and heated 

 rocks. It is moreover acknowledged that even those mineral 

 and thermal springs which, in the present state of the globe, are 

 far from volcanoes, are nevertheless observed to burst out along 

 great lines of upheaval and dislocation of rocks "*. 



It is generally admitted by geologists that igneous causes have 

 been in active operation in all past ages of the world. They have 

 been constantly changing their areas of activity on the earth's 

 surface; and those districts which are now great centres of vol- 

 canic action were, at remote geological epochs, in a state of 

 perfect tranquillity ; on the other hand, districts in which vio- 

 lent eruptions took place at former periods have often been 

 entirely free from volcanic eruptions during later geological 

 times. It would appear that the last expiring efforts of volcanic 

 action frequently manifest themselves in the form of geysers or 

 boiling springs, which, in addition to pouring forth steam and 

 water holding various salts and minerals in solution, give off 

 carbonic acid, sulphuretted hydrogen, and other gases. 



Hot and boiling springs are exceedingly numerous in some of 

 the mining regions of the Pacific States of North America, and 

 present phenomena possessing the highest degree of interest to 

 the geologist f. 



The Steamboat Springs, seven miles north-west of Virginia 

 city in the state of Nevada, are probably the most remarkable of 

 those yet discovered. They are situated at a height of 5000 

 feet above the level of the sea, near the foot of the eastern 

 declivity of the Sierra. In this locality the granite is tra- 

 versed by numerous parallel fissures, which either give out highly 

 heated water or simply eject steam. The principal group of 

 crevices, of which the direction is nearly north and south, com- 

 prises five longitudinal openings, extending nearly in a straight 

 line for a distance of more than 1160 yards. These are often 

 full of boiling water, which sometimes overflows and escapes in 

 the form of a considerable rivulet, whilst at others it does not 

 reach the surface, although violent ebullition is heard to be 

 taking place below. These waters are slightly alkaline, and 

 contain sodium carbonate, sodium sulphate, and common salt. 

 There is along the whole line an escape of carbonic acid, sul- 

 phuretted hydrogen being also evolved from certain points in 

 notable quantities. 



Silica, sulphur, and oxide of iron are deposited; and the fissures 

 are lined on either side (sometimes to the thickness of several 

 feet) by incrustations of more or less hydrated silica. This is con- 



* Elements of Geology, p. 766- 



t " Notes on the Chemical Geolog)' of the Gold-fields of California, by 

 J.Arthur Phillips/' Phil. Mag. November 1868. 



2D 2 



