TRANSITION OF SHALE INTO SERPENTINE. 405 



complete metathesis between the chemical substances in the mineral water 

 and the constituents of the shale. Such a result might be expected under 

 high pressure and at an elevated temperature. Mr. G. F. Becker has 

 spoken-'^ of metamorphic gabbro from this district in his study of the Coast 

 ranges of California. 



If, however, the gabbro is a crystalline igneous rock and forms au eruptive 

 dike, it is equally possible to explain the intermediate specimens h and c. 

 On this supposition h and c are merely shaly gabbros, whose composition has 

 been slightly changed by the mineral water which has leached out a little 

 silica, alumina and lime, w'hile an increased percentage has been effected in 

 the quantity of water, magnesia and alkalies. Furthermore, a small amount 

 of phosphate, sulphate and carbonate has been added, the iron being wholly 

 reduced in 5. 



The gap which exists between this shale and the three remaining rocks of 

 the series is too great, it would seem, to admit other than the latter view of 

 the case. For example, the sudden great decrease in silica and the increase 

 in lime and magnesia in the passage from the shale to the shaly gabbro next 

 in the series is scarcely admissible in a true transition. Field observations 

 also are adverse to the view that the second and third specimens, h and c, 

 possess any qualities of shale other than friability. 



Series II. — The specimens (except 242 and 176) embraced in this series 

 were taken from a ridge east of and above Bagley creek, at the northern 

 edge of the serpentine dike described by Mr. Turner, and were selected so 

 that each specimen w ould represent the composition of a vertical section a 

 few inches from its neighbor. 



a. Shale, friable, but not much altered and free from carbonates ; slate- 

 color ; soft. 



b. Shale, not so friable as a, but' considerably altered and containing car- 

 bonates in seams ; color more variegated. 



c. Dark-colored serpentine, almost black, containing seams of secondary 

 substance free from carbonate but ferruginous. 



d. Serpentine, almost black, with greenish tinge ; compact, with a some- 

 what bladed structure, breaking in irregular fractures ; no carbonates nor 

 chrysotile. 



Specimen c occurs at the contact between h and d, and is, according to its 

 external characters, an apparent transition between those specimens. The 

 substance used for analysis in each case was freed from the seams of car- 

 bonates. 



e. This analysis gives the composition of a bastite (number 176) which 

 properly belongs to this series. It occurs in the same dike of serpentine 



* Geology of the Quicksilver Deposits : Monograph XIII, U. S. Geol. Surv., 1888, p. 101 



