664: PROCEEDINGS OF THE BALTIMORE MEETING 



above. It is therefore clear that the statement that the pyroxene masses are 

 interlarded with older crystals, especially elongated feldspar ("ses plages, sans 

 contours exterieurs propres. sont lardes de cristaux plus anciens ; ceux de 

 feldspaths notamment s'allongent"). must be taken to include not only the tex- 

 ture,- which is poikilitic. but also the structure, in which the fine granular 

 pyroxene masses are penetrated by lath-shaped plagioclase crystals of indefi- 

 nite orientation. In other words, it includes all cases in which the elongated 

 plagioclase crystallized before the pyroxene (or other ferromagnesian) constit- 

 uent of the rock. 



At the time that Fouque and Michel Levy 21 attempted to reproduce the ophitic 

 texture artificially in 1881 they defined it as follows : 22 "As is well known, these 

 rocks are characterized by the development of microlites of triclinic feldspar, 

 molded and often inclosed by extensive areas of pyroxene." Here the large 

 areas of pyroxene are included in the definition, perhaps because this type of 

 the ophitic texture was the one actually obtained experimentally. But the 

 definition can not refer to a variety of the poikilitic texture alone, since these 

 areas either inclose or mold themselves about the earlier feldspar crystals. 

 Furthermore, the text states that the "large" areas of pyroxene had an average 

 diameter less than twice the length of the feldspar crystals. 



When Fouque and Michel Levy reported the discovery of a mineral erro- 

 neously called diamond in South African rocks, they described the inclosing 

 rocks 23 as follows : "Their type of texture, very uniform, is ophitic ; they are 

 rocks entirely crystalline, in which the feldspathic element is elongated parallel 

 to the axis a. while all the other minerals of later consolidation are granu- 

 litic." Here the important characteristic of the texture is clearly stated to be 

 the early crystallization of the feldspar with resultant automorphic elongation. 



Related Teems 



So far as the writer is aware, of all the terms that have been used as more or 

 less exactly synonymous with ophitic. ZirkeFs intersertal 24 is the only one that 

 has the right of priority as compared with Michel Levy's term. Zirkel defined 

 the intersertal structure as present in "Feldspar basalts consisting of larger 

 crystals and an apparently amorphous and not individualized matrix, pressed 

 and squeezed into the spaces between the divergent sections of the phenocrysts, 

 and so reduced in amount that it does not at all play the part of a true ground- 

 mass." Of this structure he names three varieties : First, that in which the 

 groundmass (zwischengeklemmte Masse) is wholly of glass; second, with the 

 groundmass "half glassy" by the appearance of granules in it ; third, with the 

 groundmass so abundantly supplied with needles and granules that only a little 



a Bull. Soc. Min. Fr., vol. iv, 1881, p. 277. 



22 "On sait que ces roches sont caracterisees par le developpement de microlithes de 

 feldspath tricllniques, moulds et souvent englob£s par des plages Vendues de pyroxene." 



23 C. R.. LXXXIX, 1879, p. 1125 : "Leur type de structure tres uniforme, est ophitlque ; 

 ce sont des roches entierement cristallisees, dans lesquelles 1' element feldspathique est 

 allonge' sulvant l'arete pg 1 , tandis que les autres mineraux de seconde consolidation sont 

 granulitiques." 



Si Basaltgesteine, 1870, p. Ill: "Feldspathbasalte, bestehend aus grossern Krystallen 

 und einer zwischen die divergirenden Durehschnitte derselben gedrangten und geklemm- 

 ten. als solche amorphen und-nicht individualisirten Masse, welcbe an Quantitat zuriick- 

 tretend, keineswegs die Rolle einer eigenlichen Grundmasse spielt." 



