O'T 
Of 
One of the most common types of rock that covers large areas within 
the body of syenite porphyry is represented by a specimen consisting 
almost entirely of large phenocrysts of orthoclase and microperthite. Later 
than these, and apparently replacing the larger phenocrysts, is considerable 
albite. It occurs as large and small grains, with some orthoclase and very 
little quartz, around the edges of the large feldspar crystals. Muscovite 
and green biotite altering to chlorite are also quite abundant in the inter- 
stices between the phenocrysts. Magnetite is quite abundant and calcite 
occurs as an alteration of the feldspars of the later generation. The feld- 
spar phenocrysts are much kaolinized, whereas the plagioclase of the 
groundmass is surprisingly fresh. Plagioclase and some accompanying 
calcite also occur within some of the larger phenocrysts as irregular 
blebs or quite extensive patches. 
Another phase of the syenite porphyry, somewhat less abundant, 
consists of stubby, brown feldspar crystals up to three-fourths of an inch 
in length and three-eighths of an inch in breadth, forming more than 75 per 
cent of the rock. Under the microscope the large phenocrysts are seen to 
be principally microcline and orthoclase, with some plagioclase near albite 
in composition. Large plates of a green amphibole form 5 to 10 per cent 
of the rock. Its optical properties follow. The mineral is biaxial and 
negative. The axial angle (2V) is approximately 30 degrees. The elonga- 
tion is positive. The dispersion, j A u, is moderate. The indices of refrac- 
tion are; 
a = 1*64 ± 0-005, /3 = 1-657 ± 0-002, y = 1-660 ± 0*003. 
The pleochroism is fairly strong in shades of green. The extinction, 
ZAC, varies up to 43 degrees. 
Judging from its high extinction angle this mineral seems to belong to 
the katophorite group. The latter, however, according, to Johannsen 
and Iddings, is positive, Lacroix states that katophorite may be either 
positive or negative, and that its optic angle varies from 2E — 33 to 2E = 60 
degrees. 
A rim of blue amphibole, with nearly parallel extinction, occurs around 
the edges of many of the large amphibole plates just described. It is clearly 
an alteration product, the gradation from one to the other being very 
gradual, the later blue type projecting into and from the earlier green 
amphibole in an irregular, shred-like manner. A few isolated areas of the 
blue mineral also occur well within the plates of the green variety. This is 
probably a result of action by later soda-rich solutions. Similar occurrences 
have been mentioned by TJssing 1 and Brogger. 2 The latter speaks of 
it as a zoning, giving a gradual transition from a “katoforit kern” to an 
outside rim of the character of arfvedsonite. Brogger also mentions the 
not-infrequent occurrence of an outer zone of aegirine. These occurrences 
are in solvsbergite. 
What were, apparently, either plates of amphibole or crystal of pyrox- 
ene have in some cases been altered to a mass of calcite and magnetite 
with a few irregular patches of blue amphibole, having a very small extinc- 
tion angle. 
A few fine shreds of aegirine also occur in this rock and have likewise 
been altered, around their edges, to the blue amphibole. The latter has 
apparently also formed at the expense of biotite, which is present in rela- 
UJesing, N. V.s Geology of Julianehaab, Greenland, 1911, p. 117. 
*Br62ger, W. C.: Eniptivegesteine dee Kristianiagebietee, vol. 1, p. 71 (1394). 
