100 CALIFORNIA ACADEMY OF SCIENCES. 



argue that in the region penetrated by the Forman shaft, 

 a thickness of over 1,300 feet of hornblende andesite has 

 been injected beneath an earlier mass of pyroxene andesite. 

 This I regard as a mechanical impossibility. 



Granting, for the sake of argument, the soundness of the 

 theory of laccolitic eruptions, these can occur only where 

 the overlying rock is coherent and tough. If a sheet of wet 

 paper is laid on a slab of glass or marble, it is not difficult 

 to inject beneath it a mass of water, which will simulate 

 a laccolite. But a single pin-prick in the dome allows 

 the water to escape and the paper to flatten. Eruptive 

 rocks after cooling are always cracked, and they are also 

 brittle. Laccolites cannot therefore be formed in eruptive 

 rocks. The only remaining supposition is, that the pyrox- 

 ene andesite floated upon the hornblende andesite. If solid 

 pyroxene andesite will float upon melted hornblende ande- 

 site at all, which seems improbable, the pyroxene andesite 

 would certainly not float high out of the melted mass, but 

 would be almost submerged. The hyjDothesis of flotation, 

 therefore, implies that the whole region was flooded with 

 hornblende andesite to the level of the top of Mount Kate, 

 a supposition which is entirely at variance with all appear- 

 ances. I believe also that a careful inspection of the prom- 

 ontory of augite andesite, in which the Forman shaft is sit- 

 uated, on my map, including an examination of the topog- 

 raphy, or a very hasty glance at the model prepared from 

 the map, will lead most geologists to regard the supposition 

 that the hornblende andesite has been injected beneath the 

 pyroxene andesite, as highly improbable. 



Conclusions as to Pyroxenic Rocks. — I re-assert, there- 

 fore, that there was an eruption of porphyritic pyroxene 

 rock (diabase) prior to the hornblende andesite erup- 

 tion, and that pyroxene andesites also followed the horn- 

 blende andesite. These pyroxene andesites appear divisi- 

 ble into two outflows, one of which certainly immedi- 

 ately preceded the later hornblende andesite, while there 



