i9o6,] SMITH— PARAGENESIS OF MINERALS. 225 



Spars were mostly orthoclase, andesine, oligoclase, and labradorite. 

 Of these andesine is unknown in the glaucophane rocks of Cali- 

 fornia, labradorite as a secondary mineral in only one case, — in 

 veins in the altered glaucophane-bearing gabbro of the Junction 

 School-house near Healdsburg. Orthoclase is very rare, although 

 it occurs in considerable abundance in some gneissic rocks. On the 

 other hand, albite is very rare as an original constituent of the 

 igneous rocks of the Coast Ranges, but is a very common companion 

 of glaucophane in the metamorphics. The explanation of this is 

 simple. The lime-soda feldspars are formed readily in fusions, but 

 not readily in the wet way. On the other hand, it has been demon- 

 strated by experiment that the alkali-feldspars, orthoclase and albite, 

 are formed readily in superheated solutions under pressure, and not 

 readily in magmatic fusion. Since the plagioclase molecule is unstable 

 in the presence of water in hydrothermal metamorphism, oligoclase, 

 andesine and labradorite are the first minerals to be broken up. 

 The anorthite portion of the lime-soda feldspar is set free to form 

 lime-rich minerals, either by taking up water to form lawsonite, or 

 by taking up iron and alumina to form garnet and epidote. The 

 albite portion of the molecule either joins with the ferro-magnesian 

 minerals to form glaucophane or other soda-bearing hornblendes, 

 or, if it is present in excess, crystallizes out as albite. This accounts 

 for the fact that albite is the commonest feldspar in metamorphic 

 rocks, not only in California, but all over the world, and that lawson- 

 ite is a common companion of glaucophane, in Europe as well as in 

 California. 



In the Coast Ranges are found considerable quantities of very 

 basic actinolite schists with a small amount of glaucophane. These 

 rocks appear to have been made out of pyroxenites, in which soda 

 was very scarce, for olivine, augite and orthorhombic pyroxenes 

 contain no soda. In addition to actinolite and glaucophane, the rock 

 contains large quantities of chlorite not made at the expense of the 

 actinolite, which is perfectly fresh. A mixture of about two-thirds 

 actinolite and one-third chlorite would give the composition of the 

 original pyroxenes ; for when the basic ferro-magnesian mineral is 

 broken down to form actinolite there is left a more basic portion 

 rich in magnesia to form chlorite by the addition of water. Since 



