1918] Davis: The Bacliolarkin Cherts of the Franciscan Group 277 



The iron necessary to form the glaucophane was of course already 

 present. 



The Franciscan sandstone has also locally been transformed to 

 glaucophane schist along contacts with basalt, or with serpentine. 

 The basalt itself may show development of glaucophane near the 

 contact. 17 



The sandstones contain considerable amounts of plagioclase, and 

 the soda in these feldspars may have been the source of the soda in 

 the glaucophane molecule. It is possible, therefore, that in the con- 

 tact metamorphism there was little introduction of material, but a 

 recrystallization of original constituents in a peculiar way. This was 

 the idea of J. P. Smith, 18 who concluded from his study of these meta- 

 morphic rocks that they had been formed solely by a process of re- 

 crystallization : ' ' The results all show that there is no need of sup- 

 posing that magnesian or alkaline or siliceous solutions have permeated 

 the altered rocks, adding one substance and taking away another." 



The notion of direct contribution from external sources is favored, 

 however, by the very irregular distribution of the glaucophane schists 

 around these contact zones. Also many of the contacts do not show 

 any of this type of metamorphism, a fact hard to understand in case 

 of pure thermal metamorphism. Also some rather small intrusive 

 bodies have produced larger zones of contact action than some larger 

 bodies. These peculiarities of distribution might be explained by the 

 assumption that some peculiar mineralizer was present in the emana- 

 tions from these basic rocks, under the influence of which the original 

 constituents recombined in the unusual molecular combinations which 

 are now found. 



Louderback and Sharwood 19 have presented evidence to prove that 

 in the alteration of racliolarian cherts to glaucophane schists, soda 

 was introduced from external sources. 



Distinctly favoring this idea is the study of certain rocks in San 

 Benito County by Professor Louderback. 20 Here soda-rich minerals 

 are often produced in large amounts in the contact zones of perio- 

 dotite. 



At this locality, a lenticular block of schist is included in a large 

 mass of serpentine. In the included schist there are numerous veins 

 "Ransome, ibid., p. 206, 1894. 



18 Paragenesis of the Minerals in the Glaueophane-bearing Rocks of California, 

 Proe. Am. Philos. Soc, vol. 45, p. 183, 1906. 



1 9 Crocidolite-bearing Rocks of the California Coast Ranges, Bull. Geol. Soc. 

 Am., vol. 18, p. 659, 1907. 



20 Louderback, G. D., Benitoite, its Paragenesis and Mode of Occurrence, Univ. 

 Calif. Publ., Bull. Dept. Geol., vol. 5, p. 331, 1909. 



