Lawson. J 



Matignite. 



357 



augite-syenite, laurvikite, of Brogger.* It differs rather radically, 

 however, from that type of rock in being more basic, and in hav- 

 ing a lower proportion of alumina, and higher proportions of lime 

 and ferric oxide. These chemical differences find their mineralogi- 

 cal expression in the abundance of melanite. The somewhat higher 

 proportion of magnesia is to be correlated with a larger proportion 

 of pyroxene than is found in laurvikite. The structure of the rock, 

 finally, is quite different from that of laurvikite. 



Minor Fades. — The minor facies of thegarnet-pyroxene-malignite, 

 which occurs at the northwest corner of the southwest end of the 

 lake, differs in appearance from the prevailing type, from the fact 

 that the feldspar is more abundant, and is very much smaller in size, 

 so that the rock presents a more uniformly gray color, and no strik- 

 ing contrast between the porphyritic constituents and the matrix. 

 The feldspars, however, have the same platy habit, and exhibit a feeble 

 tendency to parallelism. Those which are porphyritic have rather 

 poorly defined boundaries, and they grade insensibly into the feld- 

 spar of the matrix. The structure of the rock as a whole may be 

 fairly described as hypidiomorphic granular, although the tendency 

 to platy habit extends to all the feldspar of the rock. Interlocked 

 with the feldspar in allotriomorphic relation are melanite, segerine- 

 augite and biotite. Titanite and apatite are prominent, as before, but 

 are sometimes also allotriomorphic. The melanite never shows 

 crystal boundaries. Mineralogically the facies presents a distinction 

 from the prevailing type in the greater proportions of melanite and 

 biotite which are present, and the less proportion of aegerine-augite. 

 Its chemical character has not been fully determined, but a partial 

 examination of the rock kindly made for the writer by Miss S. Sharpe, 

 in the chemical laboratory of the University of California, shows 

 that it is somewhat more basic than the prevailing facies. This ex- 

 amination showed the presence of SiO, 48.86, A1 2 3 17.40, Fe. 2 3 

 7.67, FeO .48, Na.,0 8.13, K 2 2.8 r. The specific gravity of the 

 rock is 2.93. 



AMPH I BOLE— M A 1. 1 G N 1 TE. 



Mineralogicdl Composition . — That portion of the mass which is 



*Die Mineralien der Syenit-pegmatitgiinge, ttc. X. f. K. u. M- 16, 1890. 



