228 SMITH— PARAGENESIS OF MINERALS. [Octobers, 



are known as incrustations or reaction zones on corundum, having 

 been formed by the adjacent feldspars taking up alumina directly 

 from the corundum. And muscovite, paragonite and margarite are 

 all common as kelyphite rings or reaction zones around garnets, 

 accompanied by chlorite. In this case the feldspar molecule has 

 robbed the garnet of part of its alumina, forming mica, and the 

 residue, richer in magnesia, has taken up water to form chlorite. 

 This phenomenon is especially common in the eclogites. 



The potash-rich igneous rocks have given rise to metamorphics 

 with muscovite, the soda-rich original rocks have formed meta- 

 morphics with paragonite, and the lime-rich igneous rocks have 

 gone over into schists or eclogites with margarite. Naturally the 

 more acid rocks usually have more muscovite, and the more basic 

 ones more paragonite or margarite. 



Cyanite has been described as abundant in the European glauco- 

 phane schists and eclogites, and its possible mode of formation is 

 shown by the reactions : 



2NaAlSi308 (albite)-=4SiOo + NaoSi03 + ALSi05 (cyanite). 

 CaAloSioOg (anorthite) ^CaSiOg (wollastonite) 



+ ALSi05 (cyanite). 



But wollastonite is not known to occur in the glaucophane rocks 

 anywhere, and cyanite has been found in these rocks in California 

 only in one eclogite, the wollastonite and cyanite molecules having 

 joined themselves to others, forming more complex silicates. Silli- 

 manite, which has the same composition as cyanite, is not uncommon 

 in the siliceous rocks of the glaucophane series. 



The medium basic plagioclases are common in the diabases, 

 gabbros and basic diorites of the Coast Ranges, and from these have 

 come the bulk of the glaucoph-ane-bearing schists, greenstones and 

 eclogites. When the readjustment is only partial, we have massive 

 rocks composed of partly decomposed feldspar (saussurite), uralite, 

 and carinthine, with a little sphene and epidote or zoisite. These 

 rocks are the pseudodiabases and pseudodiorites of Becker. When 

 the disintegration of the plagioclase molecule is more complete, part 

 of the anorthite molecule takes up water and crystallizes out as 

 lawsonite, while a portion of the albite molecule crystallizes out as 



