NOMENCLATURE OF KEWEENAW AN IGNEOUS ROCKS 769 



his doleritic melaphyre is a coarser basalt, or a gabbro; his ophitic 

 melaphyre is a poikiHtic and kister-mottled diabase; and his por- 

 phyrite is chiefly andesite and trachyte. 



Lane,^ in 1 898-1 906, described the Keweenawan rocks of Isle 

 Royale and northern Michigan. His melaphyre porphyrite is the 

 equivalent of Pumpelly's ''Ashbed" diabase and Irving's diabase 

 porphyrite, Lane's melaphyre ophite is an olivine diabase, luster- 

 mottled by means of poikilitic textures; his doleritic melaphyre is a 

 basalt porphyry. Lane would confine the name diabase to dike 

 rocks. His augite syenite is said to be at least in part an equivalent 

 of Bayley's quartz diabase. He uses the term "ophitic" in a narrow 

 sense, not justified by the original definition of Michel Levy,^ nor 

 by his usage. 3 He applies it to those luster- mottled rocks in which 

 single pyroxene individuals inclose several plagioclase crystals, 

 usually lath-shaped and irregularly placed. It is, thus, for Lane, a 

 variety of the poikilitic texture. In its original meaning, still com- 

 monly used by many, and adopted here, it refers to that texture of a 

 basic igneous rock produced when the plagioclase crystallizes in 

 lath-shaped forms before the pyroxene solidifies. 



A. N. Winchell,4 in 1900, described in detail a few samples of the 

 Keweenawan rocks of Minnesota. He used the new term plagio- 

 clasite for the rocks previously known usually as anorthosites. 



N. H. Winchell and U. S. Grant^ published in 1900 by far the 

 most complete accounts of the petrography of the Keweenawan 

 igneous rocks. Their nomenclature varies very little from that 

 commonly in use at present. They described practically all the 

 petrographic types of the Keweenawan previously known and added 

 some half dozen new varieties. They used diorite porphyrite or 

 diabase porphyrite to designate more or less ophitic types of andesite 

 porphyry or augite andesite porphyry. They used Wadsworth's 



1 A. C. Lane, Geol. Surv. Mich., Vol. VI, Pt. i, 1898; Ceol. Soc. Amer., Bull., 

 Vol. XIV, pp. 369, 385, 1903; J. G., Vol. XII, p. 83, 1904; Geol. Surv. Mich., Ann. 

 Rep., 1903, pp. 205, 239, 1905; Ceol. Surv. Midi., Ann. Rep., 1904, p. 113, 1905; 

 Proc. L. Sup. Mg. Inst., Vol. XII, p. 85, 1906. 



2 Bull. Soc. Geol. Fr., Vol. VI, 1878, p. 158. 



3 Mineralogie micrographique, '1879, PI. XXXVI. See also p. 153. 



4 A. N. Winchell, Amer. Geol., Vol. XXVI, pp. 151 (197), 261, 348 (1900). 



5 N. H. Winchell and U. S. Grant, G. N. H. S. Minn., Fin. Rep., Vol. V, 1900. 



