406 W. H. MELVILLE — CHEMISTRY OF THE MOUNT DIABLO ROCKS. 



from wliich the previous specimens came, and of which c? is a typical speci- 

 men. It is a dark-green rock, with well developed cleavage surfaces, and 

 contains fine seams of chrysotile. The cleavage planes are striated, and 

 possess the very characteristic luster of bastite. In a strip of weathered 

 material on the hand specimen pyroxenes are noticeable. 



/. This specimen (number 242) is an example of fresh pyroxenite which 

 occurs in the serpentine dike, and some distance west of the locality of the 

 previous specimens of this series. It apparently contains a little olivine, 

 while the principal constituents are bronzite and diallage. There is no trace 

 of any variety of serpentine to be discovered in the sample. 



The rock specimens from a to d inclusive contained some accidental 

 organic matter. This was deducted from the analyses, which were then 

 calculated on the basis of 100 per cent, for better comparison with e and/. 



Series II — Analyses of Specimens from near Bagley Creek. 



(177) (178) (179) (181) (176) (242) 



a b c d e f 



H2O at 105° C 3.95 3.39 4.51 2.81 0.94 0.05 



H2O above 105° C 4.57 6.97 9.56 10.94 12.43 0.24 



SiOj 53.65 49.14 38.53 40.50 36.57 53.25 



P2O5 0.23 0.24 trace trace 



CrjOg trace 0.41 0.33 0.54 



AljOg 17.64 16.91 14.55 0.78 0.95 2.80 



Fe^Og 4.06 4.39 2.65 4.01 7.29 0.69 



FeO 3.72 3.82 4.01 2.04 0.37 5.93 



NiO trace trace trace 0.11 0.31 0.07 



MnO 0.01 0.22 0.32 0.13 0.10 0.09 



CaO 2.27 3.28 3.13 0.39 ' 0.14 16.22 



MgO 5.15 5.43 21.79 37.43 40.27 19.91 



Na^O 2.53 4.67 0.07 0.28 0.31 0.19 



K,0 2.22 1.53 0.88 0.16 trace trace 



100.00 99.99 100.00 99.99 100.01 99.98 



The best transition by far, from shale into serpentine, is shown in this 

 table of percentage composition. Specimen marked c occupies an interest- 

 ing position in the series, and although there exists between certain constit- 

 uents great deviations on either hand, yet the tendency of the shale h and 

 serpentine d to approach this mean is very striking. The most important 

 constituent is magnesia, and it increases regularly by about 15 per cent., as 

 it should, from h to d. Specimen c then partakes of the composition both of 

 shale and serpentine. 



There are two cases to consider in explanation of the product of change 



