642 REVIEWS 



with a knife, and sometimes as quartz rock which resembles vein quartz. 

 They vary from those which are amorphous silica to those which are 

 chiefly holocrystalline aggregates of quartz granules. The mineralog- 

 ical character of the amorphous silica is doubtful. It is perfectly 

 isotropic, but contains a less amount of water and is harder than opal. 

 Concerning the origin of the Radiolarian cherts, Professor Lawson 

 advances the theory that they are chemical precipitates from springs, 

 and supports the theory by the fact that they occur locally and in lens- 

 like masses. In working over the field in San Francisco and vicinity, 

 it has seemed to the present writer that these rocks were once wide- 

 spread. This is indicated by the small fragments of chert almost 

 everywhere found on the surface. Their present local occurrence is 

 not remarkable when it is remembered that they have suffered at least 

 two periods of erosion — that preceding the deposition of the Merced 

 series, and the present one. There is also some evidence of an 

 unconformity between the lower portion of this series and the bed of 

 sandstone which separates it into two parts. 



The serpentine of the area occurs with the Franciscan series, and 

 is in three tracts extending from northwest to southeast parallel with 

 the strike of the rocks. One of these tracts is north of the Merced 

 Valley, and two of them south. That on the north side occurs in 

 three large masses which are described as the Presidio laccolite, the 

 Potrero laccolite, and Hunters Point laccolite. These are probably 

 expansions in a dike of serpentine which extends from Fort Point to 

 Hunters Point, a distance of about ten miles. They are in places one 

 and one-half miles wide, and reach a thickness of 500 feet. Both the 

 Presidio mass and the Potrero mass consist of two lenses of serpentine 

 separated by sandstone. 



The writer calls attention to the great number of masses of 

 "medium-grained, dark, greenish gray rock" in the serpentine of 

 Potrero and Hunters Point. These have been shown to belong to 

 the hypersthene diabases, and in their more altered forms to the 

 epidiorites, and are thought to be inclusions in the serpentine. 



The present writer is not familiar with either of the two serpentine 

 tracts on the south side of Merced Valley, but they are described as in 

 all essentials similar to that on the north side. 



Professor Lawson finds nothing from either microscopic study or 

 field observations to support the theory that this serpentine has 

 originated from sandstone, but concludes that "the various conditions 



