254 The American Geologist. ^p^'^- ^^o^. 
is overlain by a Permain conglomerate "and is hence Primary" (p. 97). 
The author assigns to the Permian certain andcsitic eruptions in the 
vicinity of Miliana, and elsewhere, analogous to the porphyritic eruptions 
of several places in Europe. 
The sedimentary rocks arc described in detail, and are illustrated 
by numerous figures showing their structural relations; and the accom- 
panying fossils are listed. The author describes the singular and ap- 
parently numerous blocks of igneous rocks, and isolated bosses, which 
are scattered in the sedimentary areas. 
"The acid rocks within the Triassic area are everywhere granites, 
composed of muscovite, amphibole, garnet of a light color, very acid 
feldspar varying from oligoclase-albite to albite ; but these species 
result at least partly from secondary actions : in fact there e.xist all pos- 
sible stages between the two feldspathic extremes. One is in the pres- 
ence of the phenomena of albitization of calci-sodic plagioclases, as in 
the case of the granites of Pelvoux studied by Termier.* By the side 
of these plagioclases thus transformed, are deposited, in the openings 
and fissures of the rock albite crystals that are evidently secondary. This 
albitization is produced indifferently in the types with micas, and in 
those, more basic, with amphibole. 
These granites have frequently undergone crushing and lamination 
which have profoundly modified their structure. (Letourneu.x, Bel Ab- 
bes, etc.) ; they have thus acquired a pseudoporphyritic structure, pre- 
senting beautiful instances of the cataclastic structure. Further, it is 
worthy of note that the albitization of the calci-sodic feldspars began, 
perhaps, before the mechanical reconstruction of the minerals of the 
rock, but that it was prolonged, without any doubt, after the influence 
of erogenic action. Finally, dynamometamorphism of these granites was 
not accompanied by the phenomena of the recrystallization of their ele- 
ments as has been described for many regions." 
It might be supposed, perhaps, that the albitization of the feldspars, 
noted by the author, is a species of the recrystallization to which refer- 
ence is made by other petrographers. 
.\mong the basic rocks Dr. Gentil has observed, several of those 
peculiarities that have been described in the Pyrenees by Lacroix. 
The Tertiary and later eruptives are not described with equal care. 
The purpose of the author having been mainly to asceitain their extent 
for the construction of his geological map, and the main succession of 
parts. 
The absence, in this volume, of a suitable presentation of the mag- 
matic and chemical relations of the igneous rocks described is explained 
by the author by the statement that such investigations are reserved 
for a future memoir. This second publication will be welcomed by all 
geologists and especially by petrographers, for Dr. Gentil, who is a 
native of Algiers, has not only spent much time in the field studies and 
in the laboratory work, but has laid a foundation for a profound discus- 
sion of some of the questions which agitate at present the petrographers 
of the world. n. h. w. 
* Comptes Rendus, 124, pp. 317-320. 
