S^ SOUTHERN CALItORNIA ACADEMY OF SCIENCES. 



marked changes in organic forms ; and finally all these changes 

 together form a rational basis for the primary divisions of 

 time." ^ 



He gives as one of the signs of Critical Periods, the birth 

 of great mountain ranges. 



This Critical Period brings its to the Tertiary or Alam- 

 malian Age, inaugurated by the bodilv upheaval of the whole 

 western half of the continent, so that the great interior Creta- 

 ceous sea, which had previously divided Xorth America into two 

 parts, was drained off, and the continent became one, so that 

 this great "Critical Period" was a continent making, as well as 

 a mountain making period, and the climatic changes were doubt- 

 less commensurate with the change in the physical geography. 



At the commencement of the Cretaceous period the sedi- 

 ments accumulated along the then Pacific shore bottom, during 

 the Jura-trias period, yielding to the lateral pressure, were 

 mashed together and swollen up into the Sierra and Cascade 

 Ranges. (Le Conte). 



This change in the topography of the Pacific Coast, marked 

 by the elevation of those ranges (which in time reached to 6.000 

 or 7,000 feet) was gradual. The rivers were therefore at first 

 smaller than now, and the region, as Hayden inferred from the 

 great fresh-water Tertiary deposits, was covered by one or more 

 -vast fresh-water lakes. (Dana). 



This change also resulted in the formation of the long 

 peninsula, and the islands which were the nuclei of the present 

 Coast Range system of this region. 



The geological evidence shows that, as I have already 

 shown, the Cretaceous sea was very deep over Southern Cali- 

 fornia, so deep that in many places there is an almost entire 

 absence of fossil organisms, thus presenting great diffiiculties to 

 the proper reading of the pages of the geological history of 

 the region. 



Over Central and Xorthern California the indications are 

 that there was a gradual rise from the abysmal depths to more 

 shallow water. 



We find in Alameda, Contra Costa, Butte, Shasta and other 

 interior counties, that the marginal bottom of the Pacific Ocean 

 teemed with molluscous animal life ; in some places the cepha- 

 lopods are represented by immense numbers of individuals of 

 many species of Ammonites, Eaculites and their contemporaries 

 in such a remarkable state of preservation that the iridescence 

 is as finely shown as in our most beautiful living shells. 



At other localities the near proximity of the dry land of the 

 period is indicated by the presence of fossil wood, some of which 



3. LeConte's "Elements of Geolog3^" 



