STRIATIONS FROM SAND-BLAST ACTION 411 



Standing like a sentinel upon the west rim of the crater, and forming 

 the northern point of the V-shaped cleft often referred to, was the mass 

 of old lava known as Petit Bonhomme. This rock-mass was directly in 

 the path of all the heavy eruption clouds. It is scored with horizontal 

 and inclined grooves on the vertical north, east, and south faces. An- 

 other rock-mass even more beautifully striated is one in the south side of 

 the Y-shaped cleft and about midway of the original vertical height of 

 the wall (see figure 1, plate 38). These two examples are nearest to the 

 center of activity and are particularly instructive, because they show 

 grooving and polishing of massive rock, the grooves being many meters 

 long, but of undetermined depth. 



The Y-shaped cleft was at the head of the old gorge of the Eiviere 

 Blanche, and that gorge was the course of many dust-laden steam-clouds 

 (the nuees ardentes of Lacroix). The origin of the first of these clouds 

 has been explained. Directly after the first great outbreak the activity 

 of the volcano manifested itself in building up out of "solid" lava (that 

 is, of original material, not of debris) a cone within the old crater. The 

 material was extremely viscous and it hardened as it rose from the conduit, 

 a process that was favored by the expansion of the contained water vapor, 

 which rapidly reduced the temperature of the mass below the point of 

 solidification. An extremely steep-sided cone or dome was the result, 

 with vertical walls and 37 degree slopes of slide rock on its southwest side 

 above the head of the old Blanche gorge. As the cone rose, the explo- 

 sions occurred most frequently and violently in the southwest section of 

 its upper part. The dust-saturated clouds therefore rolled down the steep 

 slope of the new cone, gaining velocity and force as they went. The 

 velocity attained by many of the clouds in the upper part of their course 

 (as far as Morne Lenard) was determined by angular measurements from 

 the French observatory on Morne des Cadets, only 9 kilometers distant, 

 to be as much as 50 meters per second. The cloud that swept over Saint 

 Pierre on the fatal 8th of May had a velocity of not less than 130 to 150 

 meters per second 8 kilometers from the crater, as is shown by the moment 

 of the force required to overturn the iron statue of Notre Dame de la 

 Garde on the blufT of Morne d'Orange, south of the city (Lacroix). 



Such dense clouds moving with such velocity were of course able to do 

 much erosive work. Two examples of what was done on surfaces of solid 

 andesite have been given, but illustrations of the effect produced on the 

 old tuff-agglomerate composing the major portion of the mountain were 

 much more numerous. 



About 1.8 kilometers below the Y-shaped cleft in the rim of the crater 

 the gorge of the Eiviere Blanche turns sharply westward through an angle 



