168 GEOGNOSY AND GEOLOGY... 
limestones, like oolite, marly and sandy limestone, the latter often 
accompanied by clayey, sandy, and marly masses, often colored green by 
chlorite ; also dolomite and fine grains of iron-stone. 
In England the clay (Kimmeridge clay) lies under the tie (Portland 
stone), in other places the former is replaced by sandy deposits. This 
Portland stone or Portland oolite contains species of Nerinea and Pterocera. 
c. Wealden Group. This embraces the Purbeck marble, Hastings sand, 
and Weald clay, placed by some geologists in the cretaceous system. The 
accompanying strata are marls, slate clay, bituminous. shales, marl clay, 
clay marl sandstone, iron sand, quartz rock, clay quartz, foetid limestone, 
shelly conglomerate, and argillaceous iron-stone: these contain remains of 
fluviatile animals. Beds of stone coal sometimes in considerable thickness 
are occasionally met with. In England the group attains a thickness of 
between 900 and 1000 feet. 
The Purbeck strata occupy the lowest position, and consist of limestones 
which alternate with slate clay. This limestone appears to consist almost 
entirely of the shells of Paludina and Cypris, evidently indicating a fluviatile 
origin. Between these strata are found remains of reptiles and especially 
of Chelonia. The Hastings sand lies above the Purbeck beds, and consists 
principally of a quartzose iron sand alternating with sandy clays and marls. 
Here also are found beds of iron-stone, stone coal, and numerous remains 
of extinct colossal reptiles. 
The Weald clay ,overlies the Hastings sand, and contains strata of a 
tnuish potter’s-clay, alternating with thin layers of a limestone filled with 
fossils that are very similar to those of the Purbeck marble. 
Pi. 46, fig. 12, exhibits a section of the oolitic strata as it occurs in 
England. 
The entire formation rests on the strata of the keuper (red marl) a; 
immediately on this lies the lias, 6, which supports the true Jura limestone 
with the Oxford, c, and Portland, d, strata. The fresh water formation 
(Wealden) is shown in fig. 17 of the same plate. 
In fig. 12, 1 indicates the lower lias strata, 2 the lias or gryphite limestone, 
and 3 the upper lias shale. Then follow the strata, 4, 5, 6, belonging to the 
lower oolite: after these the fuller’s-earth, 7; the Stonesfield slate, 8; great 
oolite, 9; Bradford clay, 10; Forest marble, 11; Corn-brash, 12; the Kelloway 
rocks, 13; the Oxford clay, 14; Calcareous grit, 15; Coral-rag, 16 ; Oxford 
oolite, 17; Kimmeridge clay, 18; Portland stone, 19. In jig. 17, 1 indicates 
the Purbeck marble ; 2 the Hastings sand ; and 3 the Weald clay. 
The Jura formation in France forms the figure 8, the southern ring being 
completely closed. In the centre of the ring are elevated the granite 
masses, and the trachyte of Auvergne, about which the Jurassic members 
lie like a cloak. The northern ring incloses the Paris tertiary basin, 
forming a trough in which the latter is deposited. It is entirely crossed by an 
artificial canal. The lias contracts considerably, but occupies a greater extent 
on the western borders of the Vosges. It commences to the northwest of 
Luxemberg (pl. 44, fig. 6, and pl. 46, fig. 16), and passing by this place grazes 
Metz and I Nancie bends to the eastward at Dijon, and ends near Besangon. 
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