338 Postelstia 
these together are masses of broken shale, chips 
of myriad shells and other material incident to 
shore line deposit. Such) is) the base wommnne 
Miocene sandstone. Quite steadily the frag- 
ments of shells and shale diminish in quantity 
and the rock becomes a sandstone of the light 
gray, medium grained, siliceous type, so common 
everywhere as a shore-fringing sediment. Its 
cementing material in the lowest layers is calcium 
carbonate whose origin is in the shell fragments 
of the Tertiary shore. 
Within this sandstone are many variations of 
texture and hardness. Walking over the planes 
of gentle slope which dip beneath the waters of 
the Strait of Fuca, the way is difficult because of 
ridges and mounds which are of harder and more 
unyielding rock. Between these the waves have 
washed out the softer sand, plants have loosened 
the sand grains by crowding between them their 
root-like processes to search for mineral foods 
and secure mechanical support. The alternating 
harder and softer portions afford place for 
varied work of the waves. The hard rock is 
the projecting reef on which Postelsia and its 
