208 



SCIENCE. 



[N. S. Vol. XV. No. 371. 



and western side, and Mt. Scott, too, had 

 attained its full development when the 

 principal vents of basalt opened and by a 

 series of eruptions built up the surround- 

 ing country with adnate cones upon the 

 outer slope of the rim of the lake. Then 

 followed the large eruptions of dacite 

 forming Llao Rock and the northern crest 

 of the rim to Cloud Cap. These flows oc- 

 curred during the period of glaciation of 

 Mt. Mazama, and streams of lava alter- 

 nated with streams of ice, a combination 

 which doubtless gave rise to extensive 

 floods upon the slopes filling the Valleys be- 

 low with volcanic debris from the moun- 

 tain. In connection with the eruption of 

 these viscous lavas (dacites) there were 

 great explosive eruptions of pumice, 

 spreading it for 20 miles or more across 

 the adjacent country. The explosive ac- 

 tivity of Mt. Mazama culminated in the 

 eruption of the peculiar dark pumice rich 

 in hornblende which followed the outflow 

 of the tuffaceous dacite. 



DESTRUCTION OF MT. MAZAMA— ORIGIN OF 

 THE CALDEEA. 



Then came the revolution which re- 

 moved the upper 6,000 feet of Mt. Mazama, 

 as well as a large core from its base, and 

 gave rise to the caldera. How was this 

 change produced? 



There are only two ways in which it 

 could have been effected : either by an ex- 

 plosion which blew it away, or a subsidence 

 which engulfed it. 



The occurrence of vasit quantities of 

 pumice spread for a distance of 20 miles 

 in all directions about the base of Mt. 

 Mazama is evidence of a most tremendous 

 explosive eruption at that point, an erup- 

 tion the equal of which, so far as known, 

 has not yet been found anywhere else in 

 the Cascade Range. Vast quantities of fine 

 material were blown out at the same time 

 and by drainage gathered into the sur- 



rounding valleys, which it fills to an ex- 

 tent unknown, as far as I have observed, 

 upon the slopes of any of the other great 

 volcanoes of the range.* This impressive 

 evidence shows conclusively that a late, if 

 not the final, eruption of Mt. Mazama was 

 explosive, and of such magnitude as to 

 suggest that the removal of the mountain 

 and the origin of the caldera may be 

 counted among its effects. This sugges- 

 tion, however, is not supported by the evi- 

 dence resulting from a study of the ejected 

 material and its relation to the lava flows 

 of the rim. The fine material filling the 

 valleys and the pumice throughout its 

 great area is hornblendic in character and 

 belongs to the dacites of the rim. Andesitic 

 material may be present locally, but its oc- 

 currence is exceptional. Practically the 

 whole of the material ejected by the final 

 explosion is dacite. The eruption there- 

 fore was of the usual type and not of the 

 kind which removes mountains. As far as 

 may be judged from the pumice deposits 

 in the rim, the greatest eruption of that 

 sort of material from Mt. Mazama oc- 

 curred before the extrusion of the dacite 

 of Llao Rock, and furnishes evidence that 

 the greatest explosion occurred long before 

 the destruction of Mt. Mazama. 



There is another matter of importance 

 bearing directly upon the explosive theory 

 of the caldera which renders that theory 

 wholly untenable and fully corroborates 

 the conclusion derived from a study of the 

 character and distribution of the pumice. 

 The lava exposed upon the inner slope of 

 the rim is chiefly andesite, and its relation 

 is such as to indicate that solid sheets of 

 andesitic lava formed by far the larger 

 part of Mt. Mazama. If the caldera re- 

 sulted from an explosion this mass of 

 andesitic flows would be broken to frag- 



* As far as my own observation goes, the above 

 remarks apply to Lassen Peak, Mt. Shasta, Mt. 

 Pit, Mt. Thielsen, Diamond Peak and Mt. Hood. 



