4i 6 The National Geographic Magazine 



the former termed the ' ' Falaise crater " 

 and the latter the ' ' Riviere Blanche 

 crater " — a careful consideration of the 

 evidence presented fails to show that 

 these are true centers of eruption hav- 

 ing deep sources. Great explosions 

 have occurred, however, at each of the 

 localities referred to, which have thrown 

 large quantities of dust and mud to a 

 height of several hundred feet, and sent 

 out vast volumes of steam to a height 

 of man3 r thousands of feet ; but these 

 phenomena are seemingly the same, 

 although marked by greater energy, as 

 have been observed in a large number 

 of instances on both Martinique and 

 St Vincent, where the hot dust, lapilli, 

 stones, etc., ejected from the summit 

 crater, have accumulated to a great 

 depth and been invaded by surface 

 water. While the subcraters mentioned 

 should not in my opinion be considered 

 as true volcanoes, they simulate many 

 of the phenomena attending actual 

 eruptions from deep conduits. The 

 columns of heavily dust- and mud- 

 charged steam which arise from them 

 have the convoluted or cauliflower 

 structure, and at times expand at the 

 top and take on mushroom shapes, in 

 much the same manner as do the col- 

 umns of steam, heavily charged with 

 rock fragments, that are blown into the 

 air from a primary crater. In each 

 case the proximate cause is the same, 

 namely, a steam explosion. The solid 

 material blown into the air in each in- 

 stance is also of the same nature (for 

 the reason, as it seems to me, that the 

 hot dust and stones, to which the super- 

 ficial explosions are due, were supplied 

 by the eruption of the summit crater). 

 Since the observed phenomena are so 

 similar, it may be asked, What is the 

 crucial test by which a true crater may 

 be distinguished from a pseudo-crater? 

 To formulate a definite answer to this 

 question is difficult. Perhaps the best 

 reply that can be offered is that the 

 pseudo-craters are later in the time of 



their appearance than the main erup- 

 tions which supplied the hot material 

 necessary for their production, and that 

 they occur when the topographic con- 

 ditions previous to the eruptions favored 

 the accumulation of a deep deposit of 

 hot debris. In addition, on both Mar- 

 tinique and St Vincent a complete gra- 

 dation in size and energy of the super- 

 ficial steam explosions has been ob- 

 served, ranging from small geyser-like 

 spoutings, such as have occurred at 

 hundreds of localities in valleys that 

 were deeply filled with hot debris, and 

 even on broad and comparatively smooth 

 surfaces covered with a thick sheet of 

 similar material, up to the markedly 

 energetic explosions in the valleys of 

 the Falaise and Riviere Blanche. In 

 many instances the smaller surface ex- 

 plosions have been observed to follow 

 heavy rains, and the same is true also 

 of the larger explosions referred to. The 

 larger explosions from the summit cra- 

 ter, on the other hand, are more ener- 

 getic than any that can without doubt 

 be referred to the pseudo-craters. The 

 minor eruptions from the summit cra- 

 ter, however, may be due to precisely 

 the same immediate cause as the erup- 

 tions of the pseudo-crater, namely, the 

 access of surface water to highh' heated 

 rocks, so that an apparent^' complete 

 sequence may be observed between the 

 escape of steam from hot debris and the 

 discharges from true volcanic conduits. 

 It is thus seen that the discovery of the 

 crucial test asked for is difficult, and 

 the final decision, if one is reached, 

 must rest on a judicial balancing of all 

 the evidence and the weight to be given 

 to the judgment of individual observers. 

 An instructive fact furnished by the 

 pseudo-crater (even when the larger 

 and, as some persons may think, ques- 

 tionable examples are not considered) 

 which has a bearing on the theories of 

 the ultimate causes of volcanic erup- 

 tions, is the close similarity, and in fact 

 identity, that exists between the explo- 



