166 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [NoV. 21, 



Coimbra, makes it probable that these sandstones may belong to the 

 same period and form the outer hne of secondary rocks. But the 

 different mineral character of these deposits is against the supposition. 



There are still several parts of the district north of the Tagus which 

 must be explored, before the series of the secondary formations of 

 that part of Portugal can be considered complete. The narrow line 

 of country between the schists of the Deu^a and Zezere and the 

 Jurassic chain of Monte Junto, the Serra d'Aire, &c., must be fully 

 examined to establish the connection between the limestone of Thomar 

 and the other formations described. The south bank of the Mondego 

 must also be examined, as we may there hope to find some connecting 

 links between the various members of the Jurassic series. And the 

 boundary of the secondaiy district north of Coimbra must be visited, 

 to determine whether any connection exists between the sandstones 

 of Serdao, Busaco, and Coimbra. Until these spots have been Aasited, 

 the map here offered of the northern half of Portugal can only be 

 regarded as a rough approximation. 



General Remarks. 



Throughout the district here described there is less variety of 

 mineral character than are usually seen in the same range of forma- 

 tions ; the whole consisting almost entirely either of ferrugmous sand 

 and sandstone or of com.pact limestone, with very little clay or shale. 

 The sands, whether of the tertiary, the cretaceous, or the oolitic 

 periods, have all nearly the same characters, and can hardly be di- 

 stinguished by their appearance, and most of the limestones of the 

 cretaceous, subcretaceous, and Jurassic formations have a similar 

 compact structure, white colour, and conchoidal fracture. These 

 resemblances between beds of different ages add to the difficulty of 

 the first geological survey of the countrj^, and might be pleaded in 

 excuse for many errors ; but they are also in themselves facts of 

 interest, which show that the Portuguese rocks were deposited under 

 conditions which varied little during long periods : some of these 

 conditions may be ascertained from the comparison of the characters 

 of the rocks and of the animals which have been entombed in them. 



To commence with the lower beds : the fine-grained, argillaceous 

 limestones of Montemdr, Vendas Novas and Mealhada, referred to 

 the age of the lias, seem to have been deposited in deep, quiet water : 

 Ammonites and Belemnites are abmidant in them, Brachiopoda are 

 also common, but the Lamellibranchiata are rare, and I only met 

 wdth one solitary Gasteropod of the genus Pleurotomaria. 



The calcareous series below the coal at Cape Mondego has also the 

 characters of a deep-water formation : the limestones are argillaceous 

 and alternate with fine marls ; the fossils found in them were Tei^e- 

 hratidce and Cephalopoda, the Ammonites being in extraordinary 

 abundance. 



The change from the beds just mentioned to the carboniferous 

 series resting on them is so sudden, that it must be attributed to the 

 upheaval of the bed of the sea soon after the deposition of the Cape 



