335 I'he Oligocene of the Southern United States 25 



which are best developed in Limbourg and are charadlerized by 

 Nucida compta. Otherwise the fauna is poor. The clays are 

 somewhat arenaceous and the fossils poorly preserved. One 

 rarely finds a perfect Niuiila, but the clay is filled with nacreous 

 fragments of the shells. 



The clays of Boom, The clays of Boom, which are developed 

 in the valley of the Rupel, are of great importance economically 

 in the manufadlure of bricks and tiles. At Steendorp, Tamise 

 and especially at Boom, are large quarries, the working of which 

 constitutes an important industry. The excavations have been 

 carried on at Boom so as to form a regular series of terraces. 

 Hence the quarry is strikingly like a vast coliseum. On the 

 terraces great numbers of concretions (Septaria) are found, some 

 beautifully iridescent with iron pyrites. As has been noted in 

 the preceding pages, similar concretions are extremely common 

 in the corresponding beds in northern Germany. The charadler- 

 istic fossil of these clays is Leda Deshayesiajia, which can readily 

 be colledted at any of the quarries. 



M. Delheid of Brussels has made an admirable colledlion of 

 fossils (which the writer was fortunate enough to see) from these 

 clays. The species of corals and of crustaceans were very 

 striking. Among the molluscan species, which amount to about 

 sixty, Ficula concinna Beyrich, Xenophora scrutaria Phil., Ca?icel- 

 laria evulsa Sol., Dentalium Kickxi Nyst, Perna Sandbergeri 

 Desh. and Cassis Royideleti Bast., are comparitively common. 



The Oligocene of the Paris basin. The Oligocene of the Paris 

 basin forms a broad sheet lying chiefly between the Marne and 

 Loire rivers. It extends east to Epernay, west to Chartres, south 

 to Orleans and Blois, and north to Paris. It is surrounded by 

 Eocene except on the south and southeast where Miocene and 

 Cretaceous beds occur. In the Eocene which forms the northern 

 boundary, isolated outcrops of Oligocene can be ob.served at 

 Montmorency, Ecouen and other localities. 



The literature on the geology of the Paris basin is so volum- 

 nious that reference can be made here only to those publications 

 in which many original descriptions of species are to be found. 



