bascom.] APORHYOLITES. 47 
imparting to tliem the appearance of eyes, previously mentioned. PI. 
XVIII, a, is the reproduction of a photomicrograph of a section show- 
ing this structure. 
Mieropoilcilitic structure. — The micropoikilitic structure has been 
defined in connection with the quartz-porphyries, where it was not 
infrequently a significant feature of the groundinass. It is still more 
characteristic of the aporhyolites, and is occasionally present in the 
basic eruptives. 
These irregular quartz patches, inclosing microlites of lath-shaped 
feldspars or other minerals of independent orientation, give a pro- 
nounced mottled or " patchy" appearance to the groundmass, an 
appearance which has been noted in volcanics of all ages. It has been 
observed and variously described, usually without being named, in 
quartz-porphyries, felsites, porphyrites, rhyolites, and peridotites, by 
numerous writers — Irving, 1 Williams, 2 Haworth,' Cross, 4 Iddings, 5 
Diller, 6 Lindgren, 7 Teall, 8 Harker, Brogger, 10 and Nordensk jold. 1 ' Fel- 
site from the Archean rocks of Georgia shows the same structure. I2 This 
structure is likewise present in the felsites from the neighborhood of 
Boston," as is the case also with the quartz-porphyries and felsites of 
Marblehead Neck, Massachusetts. 
An acid lava of the Keweenawan series from Minnesota, recently 
examined by the writer, shows with a high power and polarized light 
the micropoikilitic structure in perfection. 14 
While the term micropoikilitic is not restricted to a quartz -feldspar 
1 It. D. Irving, Copper bearing rocks of Lake Superior: Mon. U. S. Geol. Survey, Vol. V, 1883, pp. 
99-100, PI. XIII, figs. 13 and 14. 
*G. H. Williams, Neues Jabrbuch fur Min. u ret., Supp. Vol. II, 1882, p. 607, PI. XII, fig. 3. The 
peridotites of the Cortlandt series: Am. Jour. Sci., Vol. XXX, p. 30; Vol. XXXIII, p. 139. 
3 E. Haworth, A contribution to the Archaean geology of Missouri: Am. Geologist, 1888, Vol. I, p. 
3C8, PI. I, figs. 1 and 2; also, Crystalline rocks of Missouri: Ann. Kept. Mo. Geol. Survey, Vol. VIII, 
1894, p. 195, PI. XXII. 
4 Whitman Cross, On some eruptive rocks from Custer County, Colo. : Proc. Colo. Sci. Soc, Vol. II, 
1888, pp. 232, 242. On a series of peculiar schists near Salida, Colo. : Proc. Colo. Sci. Soc, Jan., 1893, p. 8. 
6 J. P. IddingS, The eruptive rocks of Electric Peak and Sepulchre Mountain, Yellowstone National 
Park : Twelfth Ann. Rept. U. S. Geol. Survey, pp. 589, 646. 
C J. S. Oilier, Mica-peridotite from Kentucky: Am. Jour. Sci., 3d series, Vol. XLIV, Oct., 1892, p. 
287. 
7 Waldemar Lindgren, On sodalite syenite and other rocks from Montana : Am. Jour. Sci., 3d series, 
Vol. XLV, Apr , 1893, p. 287. 
8 J. J. Harris Teall, British Petrography, 1888, p. 337. 
9 Alfred Harker, Bala Volcanic Series of Rooks, pp. 23, 53, 54. 
10 W. C. Brogger, Der Miueralien der Syenitpcgmatitgangeder siidnorwegischen Augit und Nephe- 
linsyenit: Groth's Zeitschrift fur Krystallographie, Vol. XVI, p. 46. 
"Otto Nbrdenskjold, Zur Kenntniss der sogen. Hallefiinta des nordostliehen Smalands: Bull. Geol. 
Inst. Upsala, Vol. I, No. 1, 1893. 
12 A section of this felsite was loaned by Prof. L. V. Pirsson. It is of especial interest in its 
great similarity to the South Mountain felsite, thereby showing the southward persistence of this 
rock type. 
K 'J. S. Diller, Felsites and assoc. rocks north of Boston: Proc. Boston Soc. Nat. Hist., Vol. XX 
Jan. 21, 1880, pp. 355-368; also Bull. Mus. Comp. Zool. Harvard Coll., whole series, Vol. VII; geol., 
scries, Vol. I. Thin sections of these felsites were kindly loaned by Mr. Diller to the Johns Hopkins 
laboratory. They have many microscopic features in common with the South Mountain rocks, and 
like them were first supposed to bo of sedimentary origin. 
l4 Tlie writer's attention was called to the presence of this structure in the Minnesota acid volcanics 
by Dr. U. S. Grant, of the Minnesota State Geoloyical Survey. 
