Ch. XVII.] 
LACUSTRINE STRATA AUVERGNE. 
233 
traced over a considerable area, we may form some idea of the 
countless number of insects and mollusca which contributed 
their integuments and shells to compose this singularly con- 
structed rock. It is unnecessary to suppose that the Phry- 
ganesB lived on the spots where their cases are now found ; 
they may have multiplied in the shallows near the margin of 
the lake; and their buoyant cases may have been drifted by a 
current far into the deep water. 
The calcareous strata of the Limagne, like the other mem- 
bers of the lacustrine formation, are for the most part hori- 
zontal, or inclined at a very slight angle, but instances of local 
dislocation are sometimes seen. At the town of Vichy, for ex- 
ample, the strata dip at an angle of between 30 and 40 degrees ; 
in an ancient quarry behind the convent of Celestines, and near 
the hot spring at the same place, the beds of limestone are seen 
first inclined at an angle of 80°, and then vertical. 
5. Gypseous marls. — More than 50 feet of thinly-laminated 
gypseous marls, exactly resembling those in the hill of Mont- 
martre, at Paris, are worked for gypsum at St. Romain, on the 
right bank of the Allier. They rest on a series of green 
cypriferous marls which alternate with grits, the united thick- 
ness of this inferior group being seen, in a vertical section on 
the banks of the river, to exceed 250 feet. 
General arrangement and origin of the fresh-water forma- 
tions of Auvergne. — The relations of the different groups 
above described cannot be learnt by the study of any one sec- 
tion, and he who sets out with the expectation of finding a 
fixed order of succession may perhaps complain that the dif- 
ferent parts of the basin give contradictory results. The 
arenaceous division, the marls and the limestone, may all be 
seen in some localities to alternate with each other, yet it can 
by no means be affirmed that there is no order of arrangement. 
The sands, sandstone, and conglomerate, constitute in general 
a littoral group ; the foliated white and green marls a con- 
temporaneous central deposit, and the limestone is for the most 
part subordinate to the newer portions of the above groups. 
