OIL OF THE SANTA MARIA DISTRICT. 
113 
ill the porous mass and they may have acted as preservatives of the 
organisms to some extent. 
As regards the age of the oil, it is stated by F. W. Clarke a that the 
process of formation of the oil from organic sources may not be slow, 
but, on the other hand, comparatively rapid. It is usually thought, 
however, that the process of distillation is slow and is continued 
during a long time. The petroleum in the Monterey may have been 
formed immediately after the deposit was laid down, or the pro¬ 
duction of it may be still in progress. There is evidence, however, 
in the presence of burnt shale in a Pleistocene deposit (see p. 52), 
in the old and eroded deposits of asphalt, and in the presence in cer¬ 
tain asphalt deposits of the bones of extinct Pleistocene mammals 6 
which were caught in tar springs in Pleistocene time, that much of 
the oil at least was formed in the Monterey and disseminated to the 
surface a long time ago. The accumulation and dissemination of the 
oil has probably gone on continuously ever since its first formation, 
the two processes taking place simultaneously. There may be por¬ 
tions of the formation from which the livdrocarbon content has not 
yet been extracted in the form of oil, whereas other portions may 
no longer contain any of the oil in its original disseminated condi¬ 
tion. The metamorphism that gave rise to the harder shales may 
have had the effect of driving out the oil more completely than it 
has been separated from the softer shale, and thus aided its accumu¬ 
lation, although this is conjectural. 
The general conclusion is that in the Santa Maria district the 
organic material in the Monterey shale that may have acted as the 
source of the oil was without a doubt adequate in amount for the 
production of the vast quantity of petroleum now present, and that 
the forms included in greatest abundance, the diatoms, were the 
chief source, although animals and perhaps other plants also con¬ 
tributed largely. 
PHYSICAL PROPERTIES. 
GENERAL STATEMENT. 
The Santa Maria district yields four distinct grades of petroleum, in 
addition to the heavy oil which flows from sprjngs or collects as 
asphalt deposits. These petroleums vary widely in their physical 
and chemical properties and as a consequence are utilized in many 
different ways, the lighter oils usually for refining, the heavier for 
fuel, road dressing, etc. 
The oil as it comes from the wells contains varying quantities of 
gas, often amounting to a considerable percentage. The two prod- 
a The data of geochemistry (in preparation for publication by the United States Geological Survey). 
b Bull. U. S. Geol. Survey No. 309, 1907, pp. 154-155. 
