464 PETROLOGY: G. P. MERRILL Proc. N. A. S. 
or olivine in a glass base like those of Tennasilm (fig. 8), or Cabarrus 
(Monroe) offer, as it seems to me, insuperable difficulties to any such 
theory of origin and I am unable to agree with Dr. Wahl in his, at first 
FIG. 15 
thought, lucid and satisfactory explanation. It is obviously impossible 
that such could have originated from the cooling from the surface inward 
of a molten drop. Even in the hot air of Dr. Wahl's imagination the crys- 
FIG. 16 
tallization must almost certainly have begun at the surface and progressed 
inward giving rise to an exterior crust or border as in figure 16. This 
seems almost too self-evident to need mention. One must consider such 
