Varieties of PorpJtijritc'. 71 



also abundant in the groundmass. If, as seems very probable, 

 this rock is really a Tertiary andesite, then it is related to the 

 older caniptonite in a way similar to that in which the mica- 

 trachyte of mount Catini, in Italy, described hj Rosenbusch,* 

 a^jproaches the minettes. 



The remainder of these andesitic rocks must be classified 

 principally with reference to the structure of their groundmass. 

 This is coarsest and most granular in numbers 8 and 17, where it 

 is almost granitic, though with Init little free quartz. The former 

 is extensively altered, and the latter, though less so, has its horn- 

 Ijlende and part of its feldspar phenocr^^sts changed to Ijrightly 

 polarizing epidote. 



The remaining specimens form a series for the most part holo- 

 crystalline, though exhibiting as extreme members a few ex- 

 amples of unindividualized base. Their phenocrysts of plagio- 

 clase and hornl^lende are quite the same throughout. The holo- 

 crystallinc groundmass, while differing considera1)ly in fineness, 

 is in some granular (19, 25 and 26) and in others microlitic or 

 trachytic (34, 35, 28, 10 and 11). In number 37 a well marked 

 flow structure is apparent in the arrangement of the little feldspar 

 microliths. The uncrystallized character of the groundmass is 

 most apparent in numbers 27 and 39. 



Number 38, aside from being a typical andesite like the others, 

 possesses an additional interest on account of containing numer- 

 ous rounded grains of i^orphyritic quartz surrounded by absorp- 

 tion halos or zones, like those described by Mr J. S. Diller in 

 basalt t and by Mr J. P. Iddings in basalt and other rocks.'l 

 This rock once contained an abundance of brown hornblende, 

 which is now mostly altered to green hornblende or chlorite. 

 Its groundmass is hypidiomorphic and granular. The absorp- 

 tion zones, whatever they once were, consist noAV mostly of green 

 hornblende and calcite. 



Augite-porphyrite (LabradorUe-porpJtyrite f). — Number 31 is at 

 once noticeable on account of its strong macroscopic resemblance 

 to that Avell-known type of labrador-porphyrite, the so-called 

 purfido rcrde antiro, Avhich is so common among the Roman 

 lapidaries, and which is now known to have been extensively 

 quarried for ornamental purposes by the Romans at Marathonise 



* Neues Jahrbuoh fiir Miii., 1880, ii, p. 206. 



t Am. Journ. Sci., 3d series, vol xxxiii, 1887, p. 45. 



t Bulletin IJ. S. Geological Survey, no. GO, 1890. 



