2, CALIFORNIA ACADEMY OF SCIENCES [Proc. 4TH SER. 
INTRODUCTION 
When the earlier paper on this subject was published, under 
similar title, in 1905,’ it was intended that it should be the 
first of a series of contributions to be offered at intervals as 
time and opportunity were afforded for the further study of 
the region. During its preparation it was not unforeseen that 
some of the details, or some of the applications of general 
conclusions, would subsequently require alteration or amend- 
ment, as exploration in the field should be extended farther 
and a more complete knowledge of the details should be ac- 
quired. With this in view it was nevertheless believed that 
such a contribution would be well worth while, even though 
corrections might be found necessary as the study progressed, 
since it would at least serve to stimulate investigation and thus 
tend to develop our knowledge of the subject. And this result 
has undoubtedly been attained. 
Since the publication of the former paper, the attention of 
the U. S. Geological Survey has been directed to this field; 
and a systematic study of its stratigraphic and economic fea- 
tures has been begun, which will undoubtedly add much to 
our present knowledge. 
During the two years and more since the publication of the 
earlier paper, exploration has been extended along both sides 
of the range for many miles beyond the portions that had then 
been covered, affording opportunity for more detailed work 
and for a better acquaintance with the stratigraphy and with 
the conditions under which deposition took place than was then 
possible. 
Prior to, and after the publication of the former paper, 
large collections of fossils, chiefly marine invertebrates, had 
been made from all of the formations represented. As these 
had been stored in the Academy of Sciences, they were lost 
when it was burned in the great fire of San Francisco. In- 
1 Proc. Calif. Acad. Sci. 3d ser. Geol. v. 2, no. 2, pp. 156-248. 
