584 CLARENCE N. FENNER 



that cover the floor of the valley. In most places they are restricted 

 to the valley floor and few are found on even the lower slopes of 

 the adjacent mountains, but the country within a radius of a mile 

 and a half of Novarupta forms an important exception. In this 

 area, hill and valley alike have been greatly shattered, and are 

 crossed by many steaming fissures. This includes not only the 

 portions of the valley to the east and west of Novarupta, but also 

 Baked Mountain, Broken Mountain, Falling Mountain, and some 

 of the lower slopes of Trident. This area was undoubtedly a scene 

 of the greatest activity during the eruption and is still the site of 

 many fumaroles. Evidently the strains that were here set up in 

 the outer crust have been of sufficient magnitude not only to 

 cause fissures to break through the valley floor but also to shatter 

 the adjacent mountains. In most places, however, they are so 

 restricted to the floor that this topographic depression was evi- 

 dently a controlling factor, and hence a moderate depth for their 

 place of origin is implied. The phenomena suggest what might 

 be expected from the injection of a sill under a rather small thick- 

 ness of cover. 



The fumaroles were the chief subject of investigation by 

 Dr. Allen and Dr. Zies. They made many measurements of tem- 

 peratures and collected samples of gases for analysis, and much 

 of interest may be expected when their work is completed. At 

 present the account will be confined to a slight description of a few 

 of the features of the fumaroles. In temperature they run from 

 below the boiling-point of water to a heat more than sufficient to 

 melt lead and zinc. The highest temperature found was 645° C. 

 Among the evolved gases, water usually forms more than 99 per 

 cent. The remainder is mostly hydrogen sulphide, nitrogen, 

 carbon dioxide, and methane. Hydrochloric and other acids are 

 probably present, though in small amounts. 



Around the vents sulphur is often found in quantities as a 

 sublimation product, and pyrite in finely divided form is very 

 common. Ammonium chloride also has been collected, as well 

 as crystallized hematite and magnetite, and study of the crusts 

 brought home will probably reveal other fumarolic sublimates. 

 The vents are frequently alined along fissures half a mile to a mile 



