VOLCANIC ERUl'TFONS ON MAKTINK^IfE AND ST. VIN(^ENT. 843 



t'oi'iii of ;iii a\ alaiiche of rcd-liot dust. It is a lava Mown to pieces by 

 the ex])ansion of the y-as(\s it contains. It rushes down the slopes of 

 the hill, carrying- with it a teri'itic blast, which mows down e\'(>rvthin<>- 

 in its path. The mixture of dust and gases behaves in many ways 

 like a fluid. The exact chemical composition of these o-ases remains 

 unsettled. They ap})arently consist principally of steam and sulphurous 

 acid." * ■• * 



The account just ([uoted of a typical (lown-l)last from a volcano, 

 seen under faxorable conditions by trained ot)ser\ers. is perhaps the 

 best evidence on record from which to judo'e of the nature of certain 

 phases of several of the recent eruptions. In the instance cited there 

 does not seem to ha\'e l)een a lofty column of dust-charg-ed steam 

 standing- al)ove the summit of the mountain wdiicli deliected the 

 upward 1)last from the vertical conduit of tlu^ volcano, but. owing to 

 the density of the mixture of steam ;ind dust driven out, it o\'ertlowed 

 the lip of the crater and rolled down tlie mountain side. That is one 

 condition, as previously stated, which may bring about a marked vari- 

 ation in tli(> nature of an erui)tion, and in fact furnish the chief con- 

 trol t)f the secondary phenomena — the density with which the steam 

 extruded is charged with solid matter. This condition may obtain 

 control even when the explosi\'e violence is not enough to drive the 

 dust-ladi'n steam to a great height. The degree' of comminution would 

 no dou))t be another factor influencing the result. The tiner the solid 

 material was connniimted, the more fluid-like would be th(^ mixture. 



In an eruption like that descri}:)ed al)ove the topogra})hy may exert 

 a decided influence. During cnniptions of great, but of not the maxi- 

 nuun, intensity the deep notch in the southwest })()rtioii of the rim of 

 the active crater of Mont Pelee would give direction to the escaping 

 dust-charged steam and determine the course the expanding avalanche 

 would take. In the absence of such a notch, as in the case of La Sou- 

 friere, the ONcrflow would be radial. A more intense eruption from 

 Mont Pelee might also be radial, tlie notch in its riiu failing to influ- 

 ence so completely the direction takcMi l)y the greater discharge. This 

 is what seems to have occurred during the later eruptions of the vol- 

 cano, when Morne Rouge and other \illages were destroyed; but in 

 addition, the grow^th of the cone of eruption al)ove the' rim of its 

 encircling crater destro3'ed the influence of the vents, and the later 

 eruption spread in all directions in the same manner as on St. Vincent. 



3[ud f(ji/:s. — The \'alleys on the lower slopes of both Mont Pelee 

 and La Soufriere have, in numerous instances, been filled to a depth of 

 40 to 60 or more feet with hot dust and stones. The streams have 

 thus been displaced and are striving to regain their right of wa}', but 

 as 3'et, owing largely to the washing down of dust, lapilli, etc., from 

 bordering slopes, are making but slow progress with thidr work. lu 

 many instancies, in fact, the high-grade rills are bi-inging to the main 

 drainage channels more debris than the master streams can remove, 



