Lawson ] 



Geology of Cannelo Bay. 



25 



ture. The infusorial beds at Monterey appear, therefore, to be 

 exceptional and not representative of the series as a whole. 



Moreover, it becomes probable from a microscopic examination 

 that the mass of the white chalky shale, in part at least, is the fine 

 ash of a very acid volcanic eruption. The bulk of the rock is a 

 finely granular, seemingly homogeneous, cloudy, isotropic substance, 

 which in some cases shows an irregular, angular, reticulated aspect ; 

 as if composed of .fragments of glass cemented by an isotropic paste. 

 Through this, as a ground-mass, are scattered broken (never water 

 worn) crystals of quartz, orthoclase, plagioclase, and biotite, together 

 with a small proportion of fragments of hornblende. There are also 

 some minute crystal fragments of indeterminate minerals. In some 

 cases there may be observed a little secondary chalcedony. The 

 opaline beds differ microscopically from the prevalent chalky rock 

 only in that the ground-mass is less granular in aspect. The frag- 

 ments of crystals of quartz, feldspar, etc., which are scattered through 

 it are fewer in number than in the chalky rock. The suggestion of 

 the volcanic origin of the shale of the Monterey series yielded by 

 the microscope is strengthened by a chemical analysis of a repre- 

 sentative specimen of the chalky rock. The results of the analysis 

 are as follows: — 



Si 0, 



86.89 



Al, 3 



2.32 



Fe, : , 



1.28 



Ca 



043 



Mg 



trace 



K, O 



1 .26 



Na s O 



2.32 



Ignition 



4.89 



Total 



99-39 



The specific gravity varies from 1.8 to 2. 1 , a result which is 

 probably too low on account of the porous character of the rock, 

 and the consequent retention of air in its interior portions. 



The character of the ground-mass of the shale and of the opaline 

 beds also renders its determination as of volcanic or of organic ori- 

 gin difficult. It is a homogeneous but granular and cloudy iso- 



