Petrological Abstracts and Reviews 



Edited by ALBERT JOHANNSEN 



Lacroix, a. "Les roches grenues, intrusives dans les breches 

 basaltiques de la Reunion: leur importance pour I'interpreta- 

 tion de I'origine des enclaves homeogenes des roches volca- 

 niques," Comptes rendus, CLIV, No. lo (191 2), pp. 630-35. 



Basaltic breccias on the island of Reunion, about 300 miles east of 

 Madagascar, are cut by dikes and sills of syenite, gabbro, and peridotite, 

 clearly exposed in mountain amphitheaters. 



The prevailing type of syenite consists for the most part of various 

 alkali-feldspars, accompanied by alkaline pyroxenes and amphiboles. 

 A scarcer variety contains biotite and a little plagioclase. The gabbros 

 vary in composition: some contain both augite and olivine; those with 

 olivine alone pass to a variety with very basic plagioclase and finally to 

 peridotite; some of the gabbro is essexitic in character. The peridotite 

 is chiefly dunite, but this passes to wehrlite by addition of diopside. 

 The most basic of the intrusives are the oldest. Their chemical relation 

 to the volcanic rocks is not known, except that some of the gabbro is 

 almost identical in composition with some of the basalt. 



This occurrence affords a striking proof that coarsely granular rocks 

 have solidified at a depth of only a few hundred meters, in volcanic rocks 

 of Tertiary — probably late Tertiary — age. The author infers that the 

 influence of depth on crystallization has been greatly exaggerated, and 

 that, while a thick cover may be favorable, it is by no means essential 

 to the development of granular texture. He believes that granular rocks 

 may be crystallizing at the present time in the flanks of active volcanoes. 

 In the Reunion locality, he sees no confirmation of Harker's hypothesis 

 that the normal order of igneous manifestations is: (i) volcanic action, 

 (2) plutonic intrusions, (3) small intrusions. He considers the volcanic 

 rocks and the small intrusions contemporaneous. 



The author also sees in this locality a demonstration that certain 

 "homogeneous inclusions" in volcanic rocks have been formed by differ- 

 entiation of the magma prior to eruption, with the result that distinct 

 geologic bodies are formed, fragments of which are loosened and brought 

 to the surface by the ascending lava. 



F. C. Calkins 



437 



