404 W. H. MELVILLE CHEMISTRY OF THE MOUNT DIABLO ROCKS. 



c. Shaly gabbro, bearing carbonates and sulphates ; it is friable, light 

 green, resinous, and possesses a tendency to fibrous structure. Macroscopi- 

 cally, it looks like a true serpentine. 



d. Crystalline gabbro, apparently eruptive, taken at the exact contact 

 with the preceding specimen ; it is hard, very compact ; small pearly cleavage 

 planes, with striations, are well marked ; the color is dark green. 



These specimens were taken within the space of a foot, and in the order 

 tabulated. 



Series I — Analyses of Shales and Gahhros Jroyn Bogley Canon. 



*(9) (173) (174) (175) 



ah c d 



H2Oatl05°C 3.01 2.41 2.29 1.20 



H2O above 105° 5.92 2.74 2.47 1.83 



CO2 2.35 1.89 



SO3 ._- 0.93 0.24 0.43 



Si02 56.66 45.43 45.69 47.49 



P.,05 . 0.15 0.04 0.06 trace 



AI2O3 17.64 12.55 13.30 15.81 



FegOs 0.49 1.85 1.07 



FeO 5.22 6.50 4.72 4.50 



NiO 0.06 



MnO 0.19 0.21 0.24 0.41 



CaO 1.67 12.39 13.50 15.53 



MgO 3.50 13.41 13.06 10.39 



Na20 .--. 2.17 1.71 1.36 i.l6 



K2O 2.27 0.11 trace trace 



Organic matter little little 



99.82 100.09 100.86 99.45 



If these examples are to be regarded as illustrating the passage of one 

 extreme of the series into the other, as there is some evidence, then the cause 

 of this metamorphosis is to be found in the action of solfataric waters. The 

 presence of sulphates and carbonates in specimens a, h and c becomes an im- 

 portant point, as tending to the confirmation of this assumption. The Avaters 

 at present contain sulphates, carbonates and chlorides of the alkalies and of 

 calcium and magnesium, together with a little silica, iron, carbonic acid and 

 sulphuretted hydrogen. A less hydrated substance, the gabbro, is formed, and 

 by the action of the heated waters calcium and magnesium oxides are added 

 at the same time that a quantity of silica, alumina and alkalies are leached 

 out. The sudden jump between specimens a and h in the contents of lime 

 and magnesia must be explained on the supposition of a very rapid and 



♦Numbers in pareutheses indicate the office labels of the working collection. 



