292 REGINALD A. DALY 



The upper contact was quite inaccessible, by ordinary climbing, 

 though it might, perhaps, be reached with the aid of a rope let 

 down from the top of the cliff. However, the coarse grain of the 

 holocrystalline rock and the relation of the mass to the overlying 

 ash-beds show without question that it is intrusive. The section 

 given in the cliff is that of a laccolith, with a width of 160 meters 

 and a maximum thickness of about 20 meters. The ash-beds 

 above are uparched and conformable to the upper surface of the 

 laccolith, except at the southern end, where they are cut across 

 at a low angle by the gabbro. The massive lava flows overlying 

 the ash beds are little, or not at all, deformed by the intrusion. 

 Some of the upper flows may be younger than the laccolith, but 

 it is possible that all the overlying flows are the older and that the 

 lack of deformation in them is due to the lateral crowding and 

 condensation of the loose ash-beds by the laccolithic magma. 



The gabbroid body may conceivably represent the crystallized 

 product of a subterranean lava stream of great length, but its 

 deformation of the overlying ash-beds is characteristic of laccolithic 

 intrusion and it seems just to describe the mass as a true laccolith. 



The intrusive rock is dark gray in color, and slightly porous. 

 It is porphyritic, with phenocrysts of olivine. These are so nu- 

 merous that the rock appears, at first glance, to be somewhat 

 coarsely granular. In the hand-specimen the phenocrysts appear 

 roundish, and only rarely idiomorphic. Some of them are of the 

 usual olive-green color, but most are iridescent on the surfaces of 

 fracture, with the beautiful blue, green, and bronze tints of a pea- 

 cock's feather. Though the rock in general is extremely fresh, 

 this iridescence seems to be due to an incipient alteration of the 

 olivine to serpentine, which is mixed with numerous, minute 

 grains of iron ore. 



Under the microscope, the idiomorphism of the nearly color- 

 less, pale brownish olivine is more clearly manifest. It occurs in 

 individuals reaching 4 mm. or more in length. The ground-mass 

 is composed of plagioclase, augite, ilmenite, and magnetite; the 

 chemical analysis of the rock shows that a little apatite must be 

 present, but not a single crystal of it was demonstrable in the thin 

 section. The ground-mass varies irregularly from the hypidio- 



