278 B. K. Emerson — Calcite- Prehnite Cement Bock. 



The outburst of the tuff followed immediately on the out- 

 flow of the trap sheet and many of the scoriaceous bombs 

 which first fell sank quite deeply into the latter and can be 

 seen enclosed in compact trap as at the western pavilion in 

 Mountain Park. 



The cement rock formed at the base of the tuff in waters 

 which were slightly contaminated by the materials of the sand- 

 stone, the graphite scales being especially far travelled because 

 of their indestructibility and lightness. It increased to very 

 considerable thickness between the trap fragments and crystal- 

 lized so rapidly and in such high temperature that it enclosed 

 pear-shaped steam holes like a scoria. After the temperature 

 had fallen below the solution point of prehnite, these cavities 

 were filled by calcite. 



Amherst College, Mass. 



