666 PROCEEDINGS OF THE BALTIMORE MEETING 



"The structures of diabase rocks, in spite of many chiefly local and not widespread 

 peculiarities, show a certain monotony. If one considers first the structure of the dia- 

 bases according to its chief characteristic it belongs with certainty to the hypidiomor- 

 phic granular type ; in comparison with typical [bathylith- or] stock-shaped abyssal 

 rocks they present, however, a number of differing relationships to which consequently 

 nearly all students of the diabase structure have granted a separate designation, and 

 have named 'ophitic' (Fouque" and Michel Levy), 'divergent-strahlig-kornig' (Lossen), or 

 diabasic-granular. These peculiarities may be traced back to two causes ; the usually 

 very distinctly lath-like shape of the plagioclases, and the earlier, or at least not dis- 

 tinctly later, crystallization of the same from the magma in comparison with the pyrox- 

 enic constituent." 



In 1901 Rosenbusch 29 used, ophitic chiefly in the narrow sense, and added an- 

 other synonym (doleritic) to the word in this sense. He also used intersertal 

 as descriptive of the texture of a rock containing little pyroxene and some glass. 



In 1886 Williams 30 proposed the term poikilitic (first spelled poicilitic, then 

 poecilitic, and finally, in 1893. poikilitic) to designate a rock structure in which 

 one mineral in large individuals envelopes smaller individuals of other minerals 

 which are not regularly arranged. It differs from ophitic in the narrow sense 

 above defined in being applicable irrespective of the nature of the inclosed 'or 

 of the inclosing mineral, and also in being applicable whether the inclosed 

 mineral have definite form or not. But as applied to one mineral inclosing 

 rounded individuals of other minerals, it should not be used, since other terms 

 have priority, notably "globulaire" of Michel Levy, 31 for the English equivalent 

 of which the writer would suggest that globulitic is to be preferred to globular. 

 The term was first applied to rounded quartz grains in a glassy base ; later its 

 meaning was extended to apply to rounded grains inclosed by other minerals. 

 For this latter sense Salomon's contact 32 structure and Bayley's granulitic 33 

 structure are synonyms, although they are also applicable to a finely granular 

 structure made up almost entirely of small rounded grains, a grain of one min- 

 eral sometimes inclosing one or more of another mineral. 



SUMMAEY 



To summarize : a texture exists quite commonly in rather basic igneous rocks 

 which has received many names. It is a texture characterized by the fact that 

 the plagioclase feldspar, contrary to the usual order of crystallization, consoli- 

 dated in lath-shaped forms before the ferro-magnesian constituents. This tex- 

 ture was defined and illustrated by Michel Levy in 1877, when he declared it was 

 characteristic of the group of rocks he called ophites. Since then the term 



29 Elem. Gest., 2 auflage. 1901, p. 32G. 



80 American Journal of Science, vol. xxxi, 1886, p. 30 ; xxxiii, 1887, p. 139. The term 

 Poikilitic was proposed by Conybeare to designate the "New Red Sandstone," or Permian 

 and Triassic (together) of England. It was used also by De La Beche, John Phillips, 

 and H. B. Woodward in the same sense. T. H. Huxley (Geological Magazine, vol. vi, 

 1869, p. 89) suggested that It be used to refer to terrestrial deposits of Permian and 

 Triassic age, and he thought such deposits in some cases indicated continuous fauna 

 rather than changing fauna. But the use of the term in this sense is now wholly obso- 

 lete, and, even if it should be revived, it could cause no confusion with the use proposed 

 by Williams. See also : Journal of Geology, vol. i, 1893, p. 176. 



» Bull. Soc. Geol. Fr., vol. ill, 1875, p. 199. 



M Z. d. d. geol. Gesell., vol. xlil, 1890, pp. 487, 511. 



M Journal of Geology, vol. Ill, 1895, p. 1. 



