120 GEOLOGY OF THE COM STOCK LODE. 



andesites. They are accompanied by particles of calcite, and by cavities 

 which entirely resemble them in outline and general character. While 

 primitive fluid inclusions are either negative crystals, or more or less dis- 

 torted vesicles, and are bounded by smooth curves of greater or less com- 

 plexity, these inclusions, as a rule, sliow irregular edges composed of broken 

 lines. It is of course necessary that these inclusions should at some time 

 have had a connection with the minute water channels of the rock mass 

 through capillary fissures, but it by no means follows that these would 

 appear even under high powers if open, and nothing is more probable than 

 that they should often be closed by decomposition products. I have but 

 rarely observed an active bubble in inclusions of this class. 



Similar inclusions have been observed in the decomposed andesites 

 of other localities in the Di.strict, and in the same relations to the decompo- 

 sition of the feldspars. While in typical instances it appears to me easy to 

 discriminate between primary and secondary liquid inclusions in feldspars, 

 cases may arise in which a confusion is possible. There is no reason why 

 such inclusions should not occur in the older rocks as well as in the andes- 

 ites, and indeed they appear to me to do so, especially in the quai-tz-por- 

 phyries. I have not alluded to them in describing slides of the older rocks, 

 because they are there accompanied by primitive inclusions, and it seemed 

 best to mention the subject in connection with a lock in which primitive 

 fluid inclusions are very exceptional. I have borne the matter in mind, 

 however, and have not used the presence of fluid inclusions as a diagnostic 

 point, except where their primitive character appeared certain. A secondary 

 inclusion from slide 210 is shown in Fig. 22, Plate III.' 



Slide 311. 1,200 feet northwest of Geiger Grade Toll House. 



Specimen showing disseminating hornblende. Thls is a light bluisll-gray, Ordiuary- 



looking andesite, with rather a large number of visible hornblendes. Under 

 the microscope it is remarkable from the fact that, besides the hornblendes 

 which are apparent to the unaided eye, it contains a vast number of spicula? 

 of the same mineral disseminated through the groundmass. In the thin sec- 



' Mr. C. W. Cross, in bis Studien Uber Bretonische Gesteine, Vionua, Holder, has called atteation 

 to secondaiv iliiid inclusions of a, different origin .and character. 



