OCCUEEENCE AND SUCGESSFON OF EOCKS. 205 



Canon. Mount Abbie, on the other hand, shows amphitheatrical basins, 

 Avhich are not impossibly rehcs of craters, and that mountain very likely 

 represents a separate, though unimportant, eruption. But had the rock 

 covered much more country than at present, it is almost certain that other 

 patches would have been cut off exactly as those near the Sierra Nevada 

 have been, and as various tracts of augite-andesite, quartz-porphyry, etc., 

 have been separated from one another. It has been supposed that there 

 were such patches near the Combination shaft and the new Yelloiv Jacket, 

 but the statements rest upon erroneous determinations. 



In the Sutro Tunnel the fissure s}'stem parallel to the Lode extends to 

 the younger hornblende-andesite, but though this rock, particularly near its 

 western limit, shows evidences of dynamical action, I was not able to make 

 certain of anj?- regular partings within its mass. On mere geometrical 

 grounds it could hardly be expected that the fissui'es would be traceable in 

 this rock, for at so great a distance from the Lode the logarithmic curve 

 and its asymptote sensibly coincide. 



There can be no doubt that the younger hornblende-andesite succeeded 

 the augite-andesite. In the Sutro Tunnel section it is seen directly over- 

 lying and inclosing the augitic rock, and on the divide between Mount Kate 

 and Mount Rose the augite-andesite can be traced passing horizontally be- 

 neath the trachytic-looking porphyry. The peninsular-like area near Sutro 

 Shaft III. would seem, too, to be a flow from the main body, not an inde- 

 pendent or subsidiary eruption; for the tunnel, though passing close by this 

 area, shows none of the younger rock west of Shaft II., nor is there any 

 sign of special disturbance of the augite-andesite in the tunnel near Shaft 

 III. 



Basalt. — Besides the five little patches of basalt shown on the map, 

 there is another of about the same size directly west of these, and just be- 

 yond the limits of the map. It is said that a few miles farther south there 

 are considerable areas of this rock. Two of the five occurrences shown are 

 very characteristic mesas, and the rock is in every way typical. The only 

 remarkable fact connected with it is its small extension. No general effect 

 upon the history of the District has been certainly traced to it. Though the 

 basalt comes in contact only with pre-Tertiary rocks and earlier hornblende- 



