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ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN.

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porphyries, felsites, porphrites, peridotites, and rhyolites by numerous writers.^[1] This structure was also found in the pre-Cambrian felsite of Georgia,^[2] and in felsites of the same age in the neighborhood of Boston,^[3] and from Marblehead Neck, Mass.

While the term micropoikilitic is not restricted to a quartz-feldspar intergrowth, in most of the occurrences described these have been the component minerals. In the rocks under discussion the feldspathic material is often so abundant as not to permit of the determination of the mineral character of the host. In such cases, however, a clue to the nature of the cementing material is found in its optical continuity with the porphyritical quartz. The feldspar phenocrysts, on the other hand, do not

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- R. D. Irving: Monograph V., U. S. G. S., Copper-bearing Rocks of the Lake Superior Region, pp. 99-100, Pl. XIII., Fig. 13-14, 1883.
- G. H. Williams: Neues Jahrbuch für Min., etc. B. B. II. 1882, S. 607, Pl. XII., Fig. 3.The Peridotites of the Courtland series.Am. Jour. Sc., Vol. XXX., p. 30, Vol. XXXIII., p. 139.
- E. Haworth: A Contribution to the Archæan Geology of Missouri, Am. Geol., 1888, Vol. I., p. 368, Fig. 1 and 2, Pl. I.
- Whitman Cross: On some Eruptive Rocks from Custer Co., Colorado.Proc. Col. Sc. Soc., Vol. II., 1888, pp. 232, 242.
- On a series of peculiar schists near Salida, Col.Proc. Col. Sc. Soc., Jan., 1893, p. 8.
- J. P. Iddings: The Eruptive Rocks of Electric Peak and Sepulchre Mountain, Y. N. P.12th Ann. Rep. U. S. G. S., pp. 589, 646.
- Waldemar Lindgren: A Sodalite Syenite and other Rocks from Montana, Am. Jour. Sc. (3), Vol. XLV., April, 1893, p. 287.
- J. S. Diller: Mica-peridotite from Kentucky.Am. Jour. Sc. (3), Vol. XLIV., Oct., 1892, p. 287.
- J. J. Harris Teall: British Petrography, 1888, p. 337.
- Alfred Harker: Bala Volcanic Series of Rocks, pp. 23, 53, 54.
- A. C. Brögger: Der Mineralien der Syenitpegmatitgänge der südnorwegischen Augit- und Nephelinsyenit.Groth's Zeitseh. für Krys., etc., Vol. XLV., p. 546.
- Otto Nordenskjöld: Zür Kentniss der s. g. Hälleflinta des Nördostlichen Smalands.Bull. Geo. Ins. Upsala, No. 1, Vol. I., 1893, p. 232.
↑ A section of this felsite, loaned by Professor Pirsson, possesses an interesting and striking resemblance to the South Mountain acid volcanics, and indicates the southward persistence of this rock type.
↑ Thin sections of these felsites were kindly loaned by Mr. Diller. They have many microscopic features in common with the South Mountain rocks, and like them were first referred to a sedimentary origin.J. S. Diller: Felsites and their associated Rocks north of Boston.Proc. Bos. Soc. Nat. His., Vol. XX., Jan. 21, 1880.Bull. Mus. Comp. Zoöl., Harvard College, whole series Vol. XII., Geol. series Vol. 1.

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R. D. Irving: Monograph V., U. S. G. S., Copper-bearing Rocks of the Lake Superior Region, pp. 99-100, Pl. XIII., Fig. 13-14, 1883.

G. H. Williams: Neues Jahrbuch für Min., etc. B. B. II. 1882, S. 607, Pl. XII., Fig. 3.The Peridotites of the Courtland series.Am. Jour. Sc., Vol. XXX., p. 30, Vol. XXXIII., p. 139.

E. Haworth: A Contribution to the Archæan Geology of Missouri, Am. Geol., 1888, Vol. I., p. 368, Fig. 1 and 2, Pl. I.

Whitman Cross: On some Eruptive Rocks from Custer Co., Colorado.Proc. Col. Sc. Soc., Vol. II., 1888, pp. 232, 242.

On a series of peculiar schists near Salida, Col.Proc. Col. Sc. Soc., Jan., 1893, p. 8.

J. P. Iddings: The Eruptive Rocks of Electric Peak and Sepulchre Mountain, Y. N. P.12th Ann. Rep. U. S. G. S., pp. 589, 646.

Waldemar Lindgren: A Sodalite Syenite and other Rocks from Montana, Am. Jour. Sc. (3), Vol. XLV., April, 1893, p. 287.

J. S. Diller: Mica-peridotite from Kentucky.Am. Jour. Sc. (3), Vol. XLIV., Oct., 1892, p. 287.

J. J. Harris Teall: British Petrography, 1888, p. 337.

Alfred Harker: Bala Volcanic Series of Rocks, pp. 23, 53, 54.

A. C. Brögger: Der Mineralien der Syenitpegmatitgänge der südnorwegischen Augit- und Nephelinsyenit.Groth's Zeitseh. für Krys., etc., Vol. XLV., p. 546.

Otto Nordenskjöld: Zür Kentniss der s. g. Hälleflinta des Nördostlichen Smalands.Bull. Geo. Ins. Upsala, No. 1, Vol. I., 1893, p. 232.

[2] R. D. Irving: Monograph V., U. S. G. S., Copper-bearing Rocks of the Lake Superior Region, pp. 99-100, Pl. XIII., Fig. 13-14, 1883.

[3] G. H. Williams: Neues Jahrbuch für Min., etc. B. B. II. 1882, S. 607, Pl. XII., Fig. 3.The Peridotites of the Courtland series.Am. Jour. Sc., Vol. XXX., p. 30, Vol. XXXIII., p. 139.

[4] E. Haworth: A Contribution to the Archæan Geology of Missouri, Am. Geol., 1888, Vol. I., p. 368, Fig. 1 and 2, Pl. I.

[5] Whitman Cross: On some Eruptive Rocks from Custer Co., Colorado.Proc. Col. Sc. Soc., Vol. II., 1888, pp. 232, 242.

[6] On a series of peculiar schists near Salida, Col.Proc. Col. Sc. Soc., Jan., 1893, p. 8.

[7] J. P. Iddings: The Eruptive Rocks of Electric Peak and Sepulchre Mountain, Y. N. P.12th Ann. Rep. U. S. G. S., pp. 589, 646.

[8] Waldemar Lindgren: A Sodalite Syenite and other Rocks from Montana, Am. Jour. Sc. (3), Vol. XLV., April, 1893, p. 287.

[9] J. S. Diller: Mica-peridotite from Kentucky.Am. Jour. Sc. (3), Vol. XLIV., Oct., 1892, p. 287.

[10] J. J. Harris Teall: British Petrography, 1888, p. 337.

[11] Alfred Harker: Bala Volcanic Series of Rocks, pp. 23, 53, 54.

[12] A. C. Brögger: Der Mineralien der Syenitpegmatitgänge der südnorwegischen Augit- und Nephelinsyenit.Groth's Zeitseh. für Krys., etc., Vol. XLV., p. 546.

[13] Otto Nordenskjöld: Zür Kentniss der s. g. Hälleflinta des Nördostlichen Smalands.Bull. Geo. Ins. Upsala, No. 1, Vol. I., 1893, p. 232.

[2] A section of this felsite, loaned by Professor Pirsson, possesses an interesting and striking resemblance to the South Mountain acid volcanics, and indicates the southward persistence of this rock type.

[3] Thin sections of these felsites were kindly loaned by Mr. Diller. They have many microscopic features in common with the South Mountain rocks, and like them were first referred to a sedimentary origin.J. S. Diller: Felsites and their associated Rocks north of Boston.Proc. Bos. Soc. Nat. His., Vol. XX., Jan. 21, 1880.Bull. Mus. Comp. Zoöl., Harvard College, whole series Vol. XII., Geol. series Vol. 1.

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