46 
SANTA MARIA OIL DISTRICT, CALIFORNIA. 
opaline rock of analysis No. 6 were samples taken from the same 
hand specimen within 1 inch of each other. The mass of the deposit 
from which' the specimen came was soft white shale belonging high 
in the formation and contained a rough layer, a few inches thick, of 
the harder material between two beds of the soft rock. 
The soft shale has been described in the preceding pages as ‘'unal¬ 
tered,” and in referring to the harder varieties different degrees of 
“alteration” have been mentioned, for the reason that the best 
explanation of the origin of the harder rocks appears to be that they 
are products of metamorphism of the soft variety. It is believed 
that the soft white and chocolate-colored organic shale represents 
the original state of the beds of the whole formation, and that a proc¬ 
ess of silicification and crystallization has caused the changes, this 
process having been aided possibly by structural disturbances and 
pressure. The beds of soft shale are usually found in attitudes only 
gently disturbed, whereas the harder shale is most commonly much 
folded and is invariably the component rock of folds where the 
forces have been especially intense. This fact may throw light on 
the problem of the alteration of the shale, and yet it may be simply 
the outcome of the removal of the softer portion of the formation 
in the regions of greatest uplift and disturbance. The chief agent 
in causing the change was probably infiltrating water carrying 
silica in solution. In some places the process may have been simply 
or largely infiltration in the extremely porous original shale and 
deposition of silica in the interspaces, thus giving rise to hardened 
and compacted irregular granular aggregates of the original amor¬ 
phous silica and the new crystalline silica combined, the result being 
an increase in the total percentage of silica. In more extreme cases 
the original material was probably partly taken in solution and rede- 
posited, being replaced almost entirely along bands or in spots, and the 
change being carried to a less extent along other layers and in other 
areas, or else the replacement was almost complete throughout. As 
the rock was rendered more compact in this process a shrinkage may 
have been the result, or the same volume may have been retained 
and the pores filled. That solution took place along with deposition 
seems to be shown by the almost complete destruction of the forms 
of organisms. 
It is possible that the differences in the shales may be original, the 
result of variation in the material deposited. Whole series of beds of 
different material might have been deposited, giving rise to harder, 
more siliceous rocks than the soft varieties, and the same material 
might have been locally deposited in thin beds or in lenses and 
nodules, or have been intermingled with the others to form the inter¬ 
mediate varieties. But it would be difficult to say what this mate- 
