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



since it has uot been found practicable to subdivide them into 

 separate formations, notwithstanding their great thickness and com- 

 plexity. The series consist of various alternations of sands or sand- 

 stones vrith limestone, and appears to represent, on a far larger scale 

 of development, the beds which in England lie between the chalk 

 and the oolites. 



Reaching from the Bay of Cascaes to the mouth of the Vouga, 

 these subcretaceous rocks line the coast of Portugal for about 150 

 miles, except at the sea-board extremity of the Cintra Hills and the 

 point of Cape Mondego. The breadth of country covered by them 

 varies from twenty to forty miles. Throughout this district the 

 continuity of the subcretaceous beds is occasionally interrupted ; 

 being broken through by the granite of Cintra and beds raised up in 

 contact with that rock, by several great chains of limestone belonging 

 to the oolitic series, upon which the subcretaceous beds rest uncon- 

 formably, and by several local outbursts of trap. Nevertheless the 

 subcretaceous rocks probably cover more than nine-tenths of the 

 secondary district on the north of the Tagus. 



The difficulty of subdividing this great series arises from the close 

 resemblance of the different beds of limestone, and from the faint 

 traces of bedding in the loose, incoherent, ferruginous sands. Where 

 there are no beds of limestone, whole districts consist of great masses 

 of sand and gravel, in which it is difficult to find the direction of the 

 dip or to trace any clear order of superposition ; the whole almost 

 resembling a mass of diluvial gravel. Organic remains are very rare 

 in the sandstones, but they are abundant in most of the beds of 

 limestone belonging to this series, and they offer differences by means 

 of which we obtain some idea of the relative ages of the beds seen in 

 distant parts of the district : but it will require a farther examination 

 of the country before all the difficulties connected with this formation 

 can be solved. 



The beds of limestone are most prevalent between fifteen and forty 

 miles north and west of Lisbon, and become more and more rare 

 as we proceed northward ; this may be partly due to differences in 

 the age of the beds, but is probably also owing to irregularity in 

 the deposition of the limestones, which seem to be local deposits 

 intercalated in a great arenaceous formation; as the Kentish-rag 

 limestone, in England, is a most variable and uncertain companion 

 to sandstones of about the same age as those under consideration. 



In travelling southward from the north of Portugal, the subcreta- 

 ceous beds are first seen two or three miles to the south of the Vouga : 

 their northern boundary runs from N.W. to S.E., nearly parallel to 

 the course of that river : they consist of coarse incoherent sandstones 

 and sands with little trace of bedding, and closely resemble the 

 superficial gravel with which that part of the country is covered. 

 The low plains round Aveiro are so completely covered with gravel 

 that the northern edge of the sands is concealed near that city, but 

 at Serdao they are seen dipping S.E. 5°, and resting unconformably 

 on an older formation of red sandstone that dips at the line of 

 junction S. 30°, and which is described in the sequel. 



