344 Postelsia 
may be caught showing that the-calcium car- 
bonate is in the form of large rhombohedra of 
ealcite. If the imprisoned grains of quartz 
could be removed, the calcite would show a 
texture as coarsely crystalline as is frequently 
seen in vein calcite. In other words, the calcium 
carbonate, in undergoing a molecular transition 
from the organic to the mineral stage, envelopes 
the grains of quartz and other contents of the 
concretionary mass in precisely the same form 
it would assume in being deposited as vein cal- 
cite in a coarsely crystalline mineral vein. Thus, 
there is present in these interesting structures an 
exhibition of the strength of the force of crys- 
tallization. Although exercised upon a very 
common rock constituent, the result is clearer 
than is usually seen in rock formations of any 
geologic age or type of rock. It is doubtless 
force of crystallization which underlies the nota- 
ble symmetry of these bodies. A large rhom- 
bohedron of calcite, rounded off, would have a 
form like most of the larger so-called concretions 
of this coast. Crystallization has drawn the 
calcite around these nuclei from among adjacent 
