Volcanic Tuffs of Sega,las ( Ariege ).— Lacroix. 363 
coherent, deeply gullied at its outcrops, and in the depressions 
of the surface the scoria and lava blocks are accumulated. 
Thus it is possible to witness their remarkable freshness, 
which recalls that of recent volcanic rocks. 
Tuff. Microscopic examination shows that this tuff is formed of lit¬ 
tle fragments of palagonitic glass, rich in secondary doubly-refracting 
products, of limestones or calcareous marls and of unaltered pieces of 
volcanic rocks. They are united by a cement of calcite which is more 
abundant from the top downward. Thus it passes into the lower Lias 
limestone. 
Projected blocks, (a), Scoria. The scoriae are of a reddish brown 
color. Their cavites are sometimes lined, or filled, with celadonite, cal¬ 
cite, quartz (quartzine) or with chalcedony; often also they are empty 
and the rock resembles a modern scoria. The microscopic examination 
shows several microlites of andesine and of magnetite. Sometimes the 
feldspar is silicified. 
(b) , Compact lavas. A frequent kind consists of a reddish-brown lava 
with a compact texture, enclosing large cavities poor in secondary pro¬ 
ducts: the microscopic examination shows quite numerous microlites of 
andesine, disseminated in the midst of a brown glass with ferruginous 
crystallites. Here and there are calcified casts of olivine and of pyrox¬ 
ene. Everywhere are seen the same secondary products as in the 
scoriae. 
(c) , Crystalline lavas. These lavas are referable to two categories. 
Those of the first type are white; the microscopic examination reveals 
in them labradorite microlites embraced in ophitic plates of calcite. 
Here and there in the midst of these can be seen the debris of augite 
in process of replacement. The rock contains brownish glass and large 
octahedra of magDetite; it constituted originally an ophitic labradorite 
rock. Some specimens belong to a type a little less basic, the triclinic 
feldspar being andesine. 
The second type is more rare and is rich in brown glass. It contains 
large crystals of augite with a rounded form, associated ophitically with 
labradorite. 
(d) , Limestones and volcanic bombs with a limestone nucleus. The 
numerous blocks of Triassic or lower Lias limestone do not present any 
transformation. The thin plates cut so as to show the contacts of the 
bombs with their limestone nuclei make it apparent that neither the 
sedimentary, nor yet the volcanic rock, has been modified. 
En resume , the tuffs of Segalas are composed of volcanic 
projections of labradorite; the scoria? are less basic (ande¬ 
sitic) than the lavas. The most crystalline of the ejected 
lavas present a great analogy of mineralogical composition 
and structure with the ophites of Rimont, which, contrary to 
that which happens in the case of the ophites of other regions 
in the Pyrenees, contain a glassy residue. They are labrador- 
