210 
THE GEAND GASCON DISTRICT. 
which have vast thickness in the Cretaceous and form a considerable por- 
tion of the Eocene. Beds of gypsum are also frequent, forming thin separ- 
ating layers in the shaly di-\dsions, and sulphate of lime is a very important 
ingredient of the arenaceous strata from the base of the Carboniferous to 
the summit of the Jurassic. Besides its occurrence in distinct bands of 
nearly pure gypsum, it plays the part of a cement in the sand rock, and is 
also richly disseminated in the form of selenite in the sandy shales. On the 
other hand, there is a marked absence of such rocks as clay-slate. The slaty 
structure and composition has not hitherto been observed anywhere, so far 
as I know, and though argillaceous rocks are very voluminous in the Cre- 
taceous they are charged with calcareous matter, and are very distinct from 
the ordinary clay-slates of the Appalachians. 
Thus it will be noted that while the strata are remarkably homogeneous 
in their horizontal extensions, they are very heterogeneous in vertical 
range And this heterogeneity is found not only in the chemical constitu- 
ents, but also in the texture and in the mechanical projDerties of hardness, 
compactness, and solubility. This consideration is an important one, since 
upon it depends the result which is obtained by the attack of the eroding 
elements — the achitecture of the cliffs and profiles. 
(3.) Another general fact of importance is, that during the Mesozoic 
ages the surface of deposition was maintained very nearly at sea-level 
throughout the entire province. With regard to the Carboniferous strata it 
does not yet appear that the same was true. From such meager knowledge 
as we possess, there may be some reason for the opinion that the Carbonifer- 
ous sea had a considerably greater depth during the earlier and middle 
portions of that age than during the later portion. The lower Carbonif- 
erous strata (Red Wall group) consist chiefly of limestones, and the over- 
lying lower Aubrey group corresponding to the coal measures is a series of 
sandstones of exceedingly fine texture and often gjpsiferous. There is a 
notable absence in these beds of signs of very shallow water, such as ripple 
marks, cross-bedding, coarse clastic material, and littoral remains, organic 
or otherwise. The fauna, as usual, is an unsafe guide, and must be regarded 
as non-committal. On the other hand, there is no reason to suppose that 
the depth was at all profound. It is rather by contrasting the total absence 
