Mineralogy and Petrography. 247 
The Tertiary plants from the valley of the River Buchtama, at 
the foot of the Atlas, and described in Paleontographica 
(1886-87) by J. Schmalhausen. 
PListoceNE.—Carl Ochsenius has contributed two papers 
upon the age of some parts of the South American Andes to the 
Zeitschrift der Deut. Geol. Ges. (1877). He attributes the elevation 
of the platform of Lake Titicaca to volcanic action, and assigns it 
to quaternary time. 
MINERALOGY AND PETROGRAPHY.! 
PETROGRAPHICAL News.—In an article on the contact phenomena 
presented by certain Scottish olivine diabases, cutting sandstones 
and shales, Stecher ? gives us some new and important ideas on 
the general subject of contact action. These olivine diabases are 
carefully described in all their varieties. Skeleton crystals of apa- 
tite, corroded augites, twinned plagioclase, dihexahedra of quartz, 
and various rare minerals are noted in them. The quartz shows 
anomalous action in polarized light, and is peculiar in that its hex- 
agonal sections are seen under crossed nicols to consist of a kernel 
of quartz substance, surrounded by a rim of calcite. In some in- 
stances the olivine yields analcite by alteration. The most interest- 
ing portion of the paper is that which treats of the endogenous 
changes which have taken place in the dykes under the influence of 
the intruded rocks. Although more acid on their edges and in the 
neighborhood of sandstone inclusions, it was found that only in these 
places in the dykes is there any considerable development of olivine 
in perfect crystals. At a somewhat greater distance from the contact, 
the olivine becomes more skeleton-like in form. In the centre of 
e dykes it is absent. This is accounted for by Stecher in sup- 
posing that the material of the dykes cooled quickly on its 
and thus preserved in their entirety the olivine which had already 
crystallized before the rock reached the surface of the earth. As 
the inner portion cooled more gradually, the. magma became more 
acid as it dissolved material from the sandstone and de PS 
torn from its walls, and thus re-dissolved the olivine, and then 
solidified under the changed conditions.—In contrast to the results 
obtained by Stecher, in which the effect of contact action is seen 
only in the eruptive rock, are those obtained by Greimiin his studies 
of the phenomena presented by the intrusion of the Upper-Devon- 
, Edited by Dr. W. S. ; niversity, Waterville, Maine. 
; Min. u. Feroe MIG TE tae a iA e 
Neues Jahrb. f. Min., etc., 1881, i., p. 1 
