RANSOME.] 



Geology of Angel Island. 



205 



which case they agree more closely with zoisite than any other 

 known mineral. Whether the prisms forming the greater part of 

 the fourchite groundmass were actually derived from the decom- 

 position of plagioclase, it is impossible to say, as no trace oi the 

 original mineral remains; but the fact that they do not occupy. dis- 

 tinct crystallographic areas is rather against such a hypothesis. It 

 is this uncertainty in regard to the original character of the ground- 

 mass that makes the designation of the rock as a fourchite rather 

 a tentative one. The fact that the Arkansas fourchites contain 

 porphyritic crystals of augite, while the Angel Island rock does 

 not, is also a point of difference, but would hardly seem to be an 

 essential one. It is quite possible that future investigations may 

 bring to light many non-porphyritic fourchites, in which case the 

 present typical Arkansas rock would be more aptly designated as a 

 fourchite-porphyrite. Moreover, reverting again to the ground- 

 mass, we are told that "in the typical Arkansas fourchites, J. F. 

 Williams seldom found a true glass, and he even found some plag- 

 ioclase that he regarded as secondary," * while in the fourchite dyke 

 described by Kemp and Marsters in the Lake Champlain region, 

 "the base is not always, and in fact seldom, a true glass, but con- 

 tains many colorless, minute acicular crystals of parallel extinction. 

 These are probably nepheline.t The mineral matter containing 

 them shows frequently a feeble refraction, and so, although in other 

 respects resembling glass, it is not perfectly isotropic. "| In view 

 of the foregoing variations described in the base of the typical rock 

 by the authors cited, it seems justifiable to apply the name fourchite 

 to the Angel Island occurrence, it being an intrusive rock interme- 

 diate in texture between the effusive augitites on the one hand and 

 the plutonic pyroxenites on the other. 



In considering the advisability of extending the name fourchite 

 to all facies of the intrusion, much weight was assigned to a statement 



*Kemp and Marsters, The Trap Dikes of the Lake Champlain Region, 

 Bull. U. S. G. S. No. 107 (1893), p. 36. 



f Is it not possible that the mineral here tentatively referred to nepheline is 

 really identical with the small prisms determined as zoisite in the Angel Island 

 fourchite ? 



\Ibid. ,p. 36. 



