ACID VOLCANIC ROCKS OF SOUTH MOUNTAIN. 815 



porphyries, felsites, porphrites, peridotites, and rhyolites by 

 numerous writers. 1 This structure was also found in the pre-Cam- 

 brian 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 dis- 

 cussion 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 



*R. D. Irving : Monograph V., U. S. G. S., Copper-bearing Rocks of the Lake 

 Superior Region, pp. 99-100, PI. XIII., Fig. 13-14, 1883. 



G. H. Williams : Neues Jahrbuch fur Min., etc. B. B. II. 1882, S. 607, PI. 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 Archaean Geology of Missouri, Am. Geol., 

 1888, Vol. I., p. 368, Figs. 1 and 2, PI. 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. Brogger : Der Mineralien der Syenitpegmatitgange der siidnorwegischen 

 Augit- und Nephelinsyenit. Groth's Zeitseh. fur Krys., etc., Vol. XLV., p. 546. 



Otto Nordenskjold : Ziir Kentniss der s. g. Halleflinta des Nordostlichen 

 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 south- 

 ward 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 asso- 

 ciated Rocks north of Boston. Proc. Bos. Soc. Nat. His., Vol. XX., Jan. 21, 1880. 

 Bull. Mus. Comp. Zool., Harvard College, whole series Vol. XII., Geol. series Vol. I. 



