1890.] Mineralogy and Petrography. 361 
pyroxene and hornblende andesites. The chemical composition of the 
group of plutonic rocks (represented at Electric Peak) and of the 
effusive group (at Sepulchre Mountain) is shown to be the same. The 
structure of their members and their mineral composition, however, are 
different, and these are shown by the author to be due to the different 
conditions under which the two groups solidified. The different 
mineralogical compositions of the various rocks belonging to the same 
group are likewise shown to be functions of the slight differences 
which occur in their geological environment. This affects the rate 
at which the heat escapes from the magma, and also the pressure 
which is experienced during its crystallization. These in turn 
affect the efficacy of the mineralizing agents held absorbed by the 
magma before its solidification. The mineralizing agents in turn show 
their effect upon the magma in the nature of the minerals separated 
from it. Renard > announces that the Sats of St. Thomas, in the 
Antilles, are diorites, containing pl 
The former contain oligoclase, and the ‘latter bytodnite or anorthite. 
They are both much altered. The feldspar of the diorites has in most 
cases changed into epidote and quartz; that of the diabases into epi- 
dote, chlorite and calcite. The same author ® describes the rocks of 
the island of Teneriffe as scoriaceous basalts, containing olivine and 
augite of the first consolidation. The very light color of the latter 
mineral and its well-marked polysynthetic twinning lamelle cause it 
to resemble plagioclase. The lack of plagioclase places the rock in 
the group of the limburgites. The rocks from the crater of the 
Cafiadas are also basalts, whose olivines are filled with muscovitic in- 
clusions. Large crystals of andesine present in it have an undulous 
extinction. Augite andesites and trachytes containing sodalite, augite 
and sanidine with an undulous extinction are also described. An 
interesting suite of analyses of some lower Silurian felsites from the 
southeast of Ireland enables Hatch’ to divide these rocks into potash, 
soda, and potash-soda felsites. The first group comprises felsites with 
few or no phenocrysts, while the second and third groups contain 
many porphyritic crystals of a striated feldspar in a cryptocrystalline 
aggregate of orthoclase and quartz. The phenocrysts may be albite or 
anorthoclase, while the feldspar of the ground-mass is orthoclase. The 

f hornblende, and diabases 



5 Proc. Verb. Soc. Belg. d. Geol., II., 1888, p. 212. 
6 Bull. Soc. Belg. d. Geol. Memoires, XII., 1888, p. 67. 
1 Geol. Magazine, Dec., 1889, p. 545. 
