344 



SIE A. GEIKIE ON THE TEKTIAItr 



[May 1896, 



positions of others almost effaced. The horizontal distance within 

 which the vents are crowded is probably less than half a mile, but 

 the lofty proportions of the precipice tend to lead the eye to under- 

 estimate both heights and distances. 



The agglomerate is a thoroughly volcanic rock, consisting of blocks 

 of all sizes of various basalts, among which large slags are specially 

 conspicuous, the whole being wrapped in a granular matrix of com- 

 minuted volcanic detritus. The arrangement of this material is best 

 seen in the fourth vent (PI. XV. and fig. 8). In this characteristic 



Fig. 8. — Section of the same neck as that shown in PI. XV. 



mr 



volcanic neck (b in fig. 8) the boundary walls, as laid bare on the 

 face of the precipice, are vertical, and are formed of the truncated 

 ends of the banded lavas (a a) which have been blown out at the 

 time of the formation of the orifice. The visible diameter of the 

 vent was roughly estimated by me to be about 100 yards. No 

 appreciable alteration was observed in the ends of the lavas next 

 the vent. The agglomerate is coarsest in the centre, where huge 

 blocks of slaggy lava lie imbedded in the amorphous mass of com- 

 pacted debris. On either side of this structureless central portion 

 the agglomerate is distinctly stratified from the walls towards the 

 middle, at angles of 30° to 35°. Even from a distance it can be 

 observed that the upper limit of the agglomerate is saucer-shaped, 

 the sloping sides of the depression dipping towards the centre of 

 the neck at about the same angle as the rudely stratified agglo- 

 merate underneath. From the bottom of this basin to the sea-level 

 may be a vertical distance of some 30 yards. The basin itself has 

 been filled up by three successive flows of basalt, of which the 

 first has merely overflowed the bottom, the second (d), entering 

 from the northern rim of thr* 1 basin, extends across to the southern 

 slope, while the third (e), alo F o flowing from the north, has filled up 

 the remainder of the hollow and extended completely across it. 

 The next succeeding lava (/) stretched over the site in such a way 



