1895.] Petrography. 657 
in the composition and structure of their phenocysts. At the immedi- 
ate contacts of the different rocks a commingling of their materials 
seems to have taken place. Mica has been generated in the gabbro, 
and the groundmass of the lavas has disappeared, leaving a plexus of 
small feldspar laths imbedded in a clear mosaic of quartz or of quartz 
and feldspar. 
The Metamorphism of Inclusions in Volcanic Rocks.— 
In a memoir presented to the French Academy of Sciences, Lacroix? 
gives a very full resumé of the conclusions reached by him in the study 
of the action of modern volcanic rocks on the inclusions imbedded in 
them. The conclusions are based on the results of late studies as well 
as on those reached several years ago.‘ The author finds that the 
basaltic and the feldspathic effusives act differently toward foreign 
fragments imbedded in them. The former act principally through 
their high temperature, fusing the most easily melted components of the 
inclusions, while the trachytic rocks act more effectively in producing 
mineralogical changes through the aid of the mineralizers, mainly 
water, with which they are abundantly provided. The physical and 
chemical changes suffered by the material of the inclusions are dis- 
cussed separately and fully. Often the fragments in the basalts are 
reduced by fusion to a few grains of their most resistant components, 
while the fragments in the trachytes have lost only their micaceous 
constituents by fusion. Consequently the metamorphism in the latter 
cases is supposed to have been produced at a comparatively low tem- 
perature, although the new minerals produced in number exceed by 
far those produced in the basaltic inclusions at a much higher tempera- 
ture. With respect to the effects produced on rocks in situ, it is found 
.that basaltic and trachytic lavas act alike—mainly through their heat. 
The metamorphic action in both cases is comparatively slight. The 
similarity in the effects produced by the two types of lavas in this case, 
when compared with the dissimilar effects produced upon their inclu- 
sions, is explained as a consequence of the fact that all lavas, when 
they reach the surface, lose their volatile constitutents, and so, of neces- 
sity, can affect alteration in contiguous rock masses solely by means of 
their high temperature. In other words, the alteration of inclusions is 
effected at a depth beneath the surface, while the alteration of rocks in 
situ is a surface phenomenon, 
3 Mémoires présentés à P Acad. d. Sciences de l'Institut de France, xxxi, 
tSee American Naturalist, 1894, p. 946. 
