LINOS A AND ITS ROCKS 25 



in some basalts, especially in those which contain nephelite, and has 

 been attributed by Leppla^ to the presence of this mineral. 



Microscopic characters. — In thin section the same minerals are 

 seen to be present as in the preceding type but, corresponding to the 

 megascopic characters, oHvine phenocrysts are much more numerous. 

 These are seldom euhedral, but usually present rounded outlines, 

 which are frequently irregular, though embayments, such as are often 

 ascribed to corrosion by the magma, are rare. The oHvine pheno- 

 crysts carry some small inclusions of magnetite, and are occasionally 

 slightly brown on the edges through incipient alteration. The few 

 feldspar phenocrysts are of labradorite, about Ab,Ani, thick tabular 

 in shape, and frequently spongy in the interior through the presence 

 of many inclusions of glass, augite, and magnetite. No augite pheno- 

 crysts were seen in the sections examined. 



The ground-mass is somewhat coarser than in the preceding type, 

 and the abundant, highly euhedral, labradorite tables are shorter and 

 stouter, and an intersertal arrangement is not so pronounced. Small 

 anhedra of colorless or yellowish augite and of magnetite are abundant, 

 while small anhedra of colorless oHvine are present in the ground-mass 

 to a greater extent than in the preceding type, though they must be 

 considered as but accessory ground-mass constituents. A glass base 

 seems to be present occasionally, but its amount is very small. 



Mode. — While no satisfactory estimate of the mode by Rosiwal's 

 method was possible, the relative amounts of the various minerals 

 present was readily estimated by some simple readjustments of the 

 figures shown by the norm, which are given on a later page. The 

 mode is approximately as follows : 



Labradorite (AbiArii) ...... 47 



Augite . . , . . . ... . . . II 



Olivine 28 



Magnetite (titaniferous) 10 



Apatite i 



Glass and nephelite 3 



100 

 As compared with the modes of the preceding types it is clear 

 that the most marked difference Kes in the amounts of augite and 



I A. Leppla, Zeitschrift der praktischen Geologic, Vol IX, 1901, p. 175, 



