USE OF TERM "OPHITIC" 663 



defined the structure characteristic of ophites as early as 1877, .in terms not 

 identical with those quoted by Lane ; the original definition is as follows : 19 



"The ophites are characterized hy the constant presence of diallage or of augite alter- 

 ing to diallage ; this bisilicate molds the elongated crystals of triclinic feldspar, gener- 

 ally clustered in groups, which do not deserve the name of microlites in spite of their 

 elongation and rather small dimensions ; this aggregate habitually encloses old crystals 

 of titanic iron. It is to this characteristic grouping of feldspar of recent consolidation 

 and of diallage still more recent that the ophites owe their structure intermediate be- 

 tween the granulitic and the microlitic structure, but actually more closely related to the 

 former." 



There is in this original definition no statement that a single pyroxene must 

 inclose several feldspar crystals ; there is, on the contrary, the simple statement 

 that "it is to this characteristic grouping of feldspar of recent consolidation 

 and of diallage still more recent that the ophites owe their structure." The 

 essential thing is simply that the feldspars formed before the pyroxene. Ac- 

 cording to the original definition, therefore, the word ophitic has a broad 

 meaning, applying to all cases where the plagioclase crystals formed before the 

 pyroxene, and is not to be confined to those instances where the pyroxene 

 occurs in large anhedra inclosing the feldspar in poikilitic fashion. 



Usage of Michel Levy 



Proceeding now to the usage of the author of the term, we find that in the 

 great Mineralogie Micrographique of Fouque and Michel Levy, published in 

 1879, the term is defined (page 153) as follows: 20 "The ophitic texture in which 

 the crystals of feldspar are elongated parallel to one of the sides of the face 

 010, forming thus a type grading toward the microlitic rocks." It would be 

 difficult to state more clearly a definition depending not upon two or three con- 

 ditions, but upon one alone, and that one is here stated to be the elongation of 

 the feldspar. Such elongation necessarily involves the crystallization of the 

 feldspar before the final solidification of the rock. In the same work the struc- 

 ture is illustrated by several photomicrographs, which show that the term is 

 applied to rocks in which pyroxene (or a related mineral) crystallized after 

 feldspar; they also show (plate xfxxvi, figure 2) that the pyroxene is not neces- 

 sarily in large areas, but may be in small grains ("augite en microlithes globu- 

 leux"), no one of them large enough to inclose the long lath-shaped crystals of 

 plagioclase. When Michel Levy wrote his "Structure et Classification des 

 Roches firuptives," in 1889, he referred directly to the original definition quoted 



19 Bull. Society Geologique de France, vol. vi, 1878, p. 158. Read December, 1877. 

 "Les ophites sont caract£risees par la presence constante du diallage ou d'un augite 

 passant au diallage ; ce bisilicate moule des cristaux allonges de feldspath trlclinique, 

 geneYalement grouped entre eux, et qui ne meritent pas le nom de microlithes malgre" 

 leur allongement et leur dimensions assez exigues ; le tout englobe habituellement des 

 cristaux anciens de fer titang. C'est a ce groupement caractgristlque de feldspath de 

 consolidation recente et de diallage plus r£cente encore que les ophites dolvent leur struc- 

 ture interm^diaire entre la structure granulitique et la structure microlitique, mals se 

 rattachant en realite" plus intimement a la premiere." 



20 "La structure ophitique, dans laquelle les cristaux de feldspath s'allongent suivant 

 l'un d£s c6t6s de la face g 1 (010), formant ainsi un type de passage vers les rochea 

 microlltlques." 



