University of California. 



[Vol. i. 



were areas of dark, interstitial material, full of gas-filled vesicles, 

 and perfectly isotropic. It is undoubtedly a vesicular glass, formed 

 by the fusion, and subsequent quick cooling, of the finer interstitial 

 portions, and possibly more basic constituents, of the sandstone. 

 It is fairly certain that such a glass could only result from the com- 

 paratively rapid chilling of the fused material, and this affords new 

 evidence in support of the hypothesis that the original top of the 

 serpentine dyke has here not yet been exposed by erosion. 



The small area of altered eruptive rock is mapped as if it were 

 surrounded by sandstone, in which case it is the only mass of such 

 rock found outside of the serpentine area. Its altered condition, 

 however, can be consistently explained only by the supposition that 

 the serpentine is not far away, and, owing to the manner in which 

 the relations of the rocks are concealed at this spot, it may be much 

 closer than is indicated by the mapping. 



The origin of these inclusions has proved a very puzzling ques- 

 tion. From the fact that they can frequently be observed to become 

 finer grained as the contact with the serpentine is approached, it 

 might be argued that they are intrusive into the latter rock. On 

 the other hand, these isolated masses have been considerably meta- 

 morphosed, while the adjacent serpentine shows no signs of altera- 

 tion, nor of penetration by apophyses. Lastly the distances between 

 these masses, and their complete isolation from each other, preclude 

 the idea suggested in explanation of somewhat similar occurrences,* 

 that they were once a dyke in the serpentine, which has been dis- 

 rupted, and the parts displaced by shearing. On the other hand, it 

 is very improbable that these numerous separate bosses of igneous 

 rock should have been intruded with such persistency into the 

 relatively massive and resistant serpentine, and yet have avoided so 

 completely the shattered and upturned strata on either side. The 

 only known occurrence of this rock outside of the serpentine area 

 is in the small ravine previously noted, and where it is quite proba- 

 ble that the serpentine is just below the surface. 



The hypothesis most in harmony with the observed facts is 

 that the masses of rock at present included in the serpentine, at one 

 time occupied a more or less continuous fissure, as a dyke of much 



*Geology of Tehama, Colusa, Lake, and Napa Counties, by H. W. Fair- 

 banks, nth Rept. State Mineralogist (California), p. 70. 



