University of California. 



[Vol. 3. 



rock magma does not appear to have been as yet recognized 

 among the known occurrences of rocks, and it is, therefore, 

 proposed to name it, for convenience in reference, Plumasite, 

 from Plumas county, in which it occurs. For purposes of refer- 

 ence to this type plumasite may be defined as a rock resulting 

 from the consolidation of a magma having the composition of a 

 medium acid plagioclase with an excess of alumina. The extent 

 to which the alumina is in excess is not material to the defini- 

 tion. It is characteristically a coarse allotromorphic granular 

 aggregate of acid plagioclase with idiomorphie crystals of 

 corundum, but on its chilled margins may have a fine-grained 

 porphyritic structure with phenocrysts of plagioclase in a plagio- 

 clase groundmass. 



Other Facie* of the DyTce. — At the exposure 100 feet northwest 

 of that just described the dyke has an exposed width of LI feet. 

 About two-thirds of the exposure on the southwest side consists 

 of a very coarse-textured white feldspar rock the same as that 

 above described, but without corundum so far as could be dis- 

 covered. Scattered sporadically through this coarse feldspar 

 rock there are occasional nests of a greenish gray mineral having 

 a fibrous radial habit which proves on microscopic examination 

 to have the characters of a colorless monoclinic amphibole. 

 These are apparently secondary and not original products of the 

 crystallization of the magma. The remaining third of the dyke 

 on the northwest side is much finer grained and is porphyritic in 

 structure, though apparently consisting wholly of white feldspar. 

 It appears to be the chilled selvage of the dyke, but, owing to 

 the unsatisfactory character of the outcrop, its relations to the 

 coarse-grained portion of the dyke could not be determined. 



The only other exposure of the dyke is about 25 feet to the 

 southeast of the first outcrop described. Here the rock is again 

 fine grained to microcrystalline and porphyritic. The porphyritic 

 crystals range up to 5 millimeters in size, and usually show poly- 

 synthetic twin striations on the cleavage faces. Optical reac- 

 tions show that these porphyritic plagioclases are andesine. The 

 groundmass in which these phenocrysts are involved is a very 

 fine microgranitic aggregate of feldspar whose composition is 



