Transactions . ds them to observe thatthe similarity seems to denote that the cause, which set about theformation of the spheroids, was not a chemical phenomenon. Lawson, in discussing the origin of the structure, recognizesmagma-differentiation presented in a two-fold aspect, primaryin the magma and secondary within the orbule, noting that thestructure is rhythmically recurrent. He makes an importantsuggestion in explanation of the fairly uniform size of the or- 3 Sitzungsber d. neiderrhein Ges., vol. xix., p. 185 (1862). 4 This classification which appeared originally in the Memoires de VAca
Transactions . ds them to observe thatthe similarity seems to denote that the cause, which set about theformation of the spheroids, was not a chemical phenomenon. Lawson, in discussing the origin of the structure, recognizesmagma-differentiation presented in a two-fold aspect, primaryin the magma and secondary within the orbule, noting that thestructure is rhythmically recurrent. He makes an importantsuggestion in explanation of the fairly uniform size of the or- 3 Sitzungsber d. neiderrhein Ges., vol. xix., p. 185 (1862). 4 This classification which appeared originally in the Memoires de VAcademicImperiale des Sciences de St. Petersbourg, VII. Series, tome xlii., No. 3, is quoted byMessrs. Kessler and Hamilton from a paper by F. D. Adams in Bulletin of theGeological Society of America, vol. ix., p. 170 (1898). 5 Sitzungsber d. neiderrhein Ges., vol. xix., p. 185 (1862), cited by Kessler andHamilton, American Geologist, vol. xxxiv., p. 140 (September, 1904). ORBICULAR AND CONCRETIONARY STRUCTURE. 41. Fig. 1.—Specimens of White Volcanic Tufa from Tucson, Ariz. (One-ThirdSize), Showing Concentric Nodular Structure. 42 ORBICULAR AND CONCRETIONARY STRUCTURE. bules, and the approximately uniform spacing of their centers,stating: This dimensional factor in the problem seems to in-dicate the spacial limits of osmotic currents within the timenecessary for crystallization. Dana regarded orbicular diorite as concretionary, remarkingthat solidification after fusion often produces concretions in themass. Geikie describes orbicular structure as one in whichthe component minerals of a rock have crystallized in such away as to form spheroidal aggregations sometimes with inter-nal radial or concentric grouping. Similar rocks are known to Germans as Kugel-granite, orconcretionary granite. Chrustschofr is said to regard them ascontact-phenomena and due to the crystallizing of the granitematerial about pre-existing fragments of other rocks. One ofthe more striking examples is shown
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