Peckham and Hali—Thomsonite from Minnesota. 123 
viriditic mineral. The lower layers are firm and compact, 
while the upper are extensively jointed and fractured, and 
filled with amygdaloidal cavities. These cavities, in whatever 
manner they were originally formed, have become filled with 
zeolitic minerals. Some of the cavities are now empty, but 
evidently as a result of the removal of their contents by solv- 
ents percolating through the enclosing rock. Occasionally the 
cavities are only partially filled, and the substance within 
Shows on its surface unmistakable traces of the action of solv- 
ents. In some cavities one mineral is nearly all washed away, 
leaving the surface of the remaining one or several, as the case 
may be, rough or uneven, as originally formed. This occurs 
only where water has had access. 
The prevailing mineral, thomsonite, is only sparsely distribu- 
ted in the lower and compacter beds of the forinsteon. The 
general occurrence of the several other minerals, so abundant 
here, would seem to indicate that this mineral was formed first 
the narrow beaches of this vicinity in the form of pebbles of 
various sizes frequently unbroken and beautifully polished. 
