665 



glass remains. According to its original definition, therefore, the intersertal 

 structure requires large euhedral crystals in divergent groups in a groundmass 

 which contains more or less glass. In this form it would seem to apply very 

 well to certain coarsely spherulitic textures rather than to the ophitic texture. 

 It may be said that the later usage of various writers, notably Rosenbusch and 

 Zirkel, gives the word a somewhat different meaning, making it apply to a tex- 

 ture in which rudely divergent feldspar crystals occur in a base containing more 

 or less glass. In this sense the word is later than ophitic, and therefore can be 

 regarded as nothing more than a variety of the latter, in which glass is present 

 in the groundmass ("mesostasis"). 



Other terms, which are somewhat related to ophitic in their meaning, include 

 luster-mottled, "divergent-strahlig-kornig," diabasic, doleritic, radiolitic, and 

 poikilitic. These terms are all of later origin than ophitic. Their history may 

 be summarized as follows : 



Pumpelly 25 proposed the term luster-mottlings (whence, of course, luster- 

 mottled) only a month later than the date of Michel Levy's article on ophites 

 and their structure. The term was quite fully described and applied to those 

 ophitic textures which are also poikilitic. It would probably be more com- 

 monly used if it were less cumbrous. It is an exact synonym of ophitic in the 

 narrow sense advocated by Lane. 



Lossen 26 is the author of the awkwardly long expression "divergent-strahlig- 

 kornig." He defined it in 1880 thus : 



"Especially the diabases of the horizon under discussion are accustomed to possess 

 dominantly a more or less distinct divergent radial-granular, not pure-granular, struc- 

 ture, in which the lath-shaped development of the plagioclase individuals dominates the 

 fabric, so much, indeed, that the rest of the mineral particles are arranged between the 

 meshes of the feldspar laths." 



The essential thing in this definition is the dominance of lath-shaped plagio- 

 clase crystals in the texture. The term is therefore a synonym of ophitic. 



Lcewinson-Lessing, 27 in 1887, suggested the shorter term radiolitic (radio- 

 litische) as a substitute for Lossen's term. 



In the same year Rosenbusch 2S proposed the term diabasic (diabasisch-kor- 

 nig) for the ophitic texture. His description of this texture was as follows: 



25 Proceedings of the American Academy, vol. xiii, 1878, p. 260. 



28 Jahrbuch k. pr. geologische Landesanst, 1880, p. 8 : "Speciell die Diabase des im 

 Rede stehenden Horizontes pflegen vorwiegend eine mehr minder deutlich divergent- 

 strahlige-kornige, nicht rein kornige Structur zu besitzen, wobei die leistenformige Aus- 

 bildung der Plagioklas-Individuen das Gefiige beherrscht, so zwar, dass die iibrigen Min- 

 eralgemengtheile zwischen das Maschenwerk dieser Feldspathleisten eingeordnet sind." 



27 T. M. P. M., vol. ix, 1887, p. 70. 



28 Mikroskopische Physiographie, 2 auflage, II, 1887, p. 190 : "Die Structurformen der 

 Diabasgesteine zeigen trotz manchen, meist localen und wenig verbreiten Eigenthiimlich- 

 keiten eine gewisse Monotonie. Betrachtet man zunachst die Structur der Diabase nach 

 ihren Hauptcharakter, so gehort dieselbe mit Entschiedenheit zu den hypidiomorph 

 kornigen ; im Vergleich mit den typischen stockformigen Tiefengesteinen stellen sich 

 jedoch eine Anzahl abweichender Verhaltnisse hereaus, denen zufolge fast alle Forscher 

 der Diabasstructur eine Sonderstellung einraumen und sie als 'ophitisch' (Fouque - und 

 Michel-Levy), 'divergent-strahlig-kornig' (Lossen), oder diabasisch-kornig bezeichnen. 

 Diese Eigenthiimlichkeiten lassen sich auf zwei Ursachen zuriickfuhren ; die meistens 

 sehr ausgesprochene Leistform der Plagioklase und die friihere oder doch nicht ausges- 

 prochenen spatere Krystallisation derselben aus dem Magma im Vergleich zu den pyroxe- 

 nischen Gemengtheilen." 



