3i8 



C. A^. FENNER 



study of this and of the other occurrences it is seen that the blocks 

 of sandstone are in reahty made up of finely divided material 

 caught up from the bottom of the lake and carried to their 

 present position in the form of dust by the blast of steam. 

 Later they were consolidated by the deposition of secondary 



Fig. 13 



minerals, but the amount of the latter is so small as not to be 

 perceived at a casual glance. So fine was the dust that it pene- 

 trated the 'most minute crevices and at times its presence can be 

 detected only with a lens. Fig. 13 shows a small block of trap with 

 sand inclusions, drawn in careful detail, and Fig. 14 is another portion 

 of the same block as Fig. 13, showing dust-lined amygdules, later 



