the Neighbourhood of Lisbon. 125 



of basalt are at the summit of Cabeca de Montachique to the north-west of Loures ; 

 some less perfect occur at the foot of a battery called Forte de San Joao das Maias, 

 on the bank of the Tagus, near Oeiras. 



The only beds which rest upon the basalt belong to the tertiary series ; they 

 cover its south-eastern edge, and were deposited after the cessation of the basaltic 

 eruption. The position of the basalt with regard to the secondary formations, 

 deserves particular attention. That part of the great deposit to the west of Loures, 

 breaks the continuity of the secondary strata (see PL XIV. and PI. XV. Sect. G 

 and 8), both the red sandstone and Hippurite limestone being divided by it into 

 two portions. The beds which lie to the northward of the basalt are neaily hori- 

 zontal, and are usually overlaid by that rock ; while on the southern side of the 

 basalt, the red sandstone is elevated into bold steep hills, in which the beds dip to 

 the S.E. at angles varying from 40° to 70°, and in one instance to 90°. The Hip- 

 purite limestone, resting upon the southern edge of this line of red sandstone, has 

 partaken of the same movement, and dips in the same direction from 30° to 45°, 

 the greatest elevation of both of these formations occuring in the narrow ridge of 

 hills extending from Canessas towards Loures. The basalt also rests upon the 

 southern foot of this elevated ridge of Hippurite limestone, and thence it reaches to 

 the Tagus, the elevation of the limestone gradually diminishing, and the sheet of 

 basalt thinning off to the southward, till, along the bank of the river, the basalt 

 rests upon nearly horizontal beds of the limestone (see Coast Section 3), filling 

 up hollows in the surface, and gradually diminishing in thickness till it is lost 

 near Oeiras. On the north-west of Loures the basalt spreads over the Hippurite 

 limestone (there inclined about 20°) in the same manner, and thins away toward 

 the N.E. till it totally disappears without reaching the Tagus. 



It is therefore evident, that the whole of this great mass of basalt was thrown up 

 after the deposition of the last of the secondary and before that of the oldest of the 

 tertiary formations, through a great cleft in the secondary rocks which extended 

 westward from the site of the village of Loures ; that the eruption of the basalt 

 elevated the ridge of red sandstone and Hippurite limestone to the south of the 

 cleft ; and that the whole of the rest of the deposit of basalt, ranging N.E. toward 

 Verdelha and southward to the mouth of the Tagus, flowed out through the narrow 

 gorge behind Loures, and spread itself in all directions. 



In no spot does the basalt appear to have passed the present line of the Tagus, 

 and in some parts, as already stated, it has not even reached it, thinning away oil 

 the north near Verdelha, and at the entrance to Lisbon from Campolide. 



The section in the cliff of Cascaes Bay (PI. XV. Sect. 10) aifords a good example of the disturbing 

 power of basalt. About a quarter of a mile cast of Cascaes a perpendicular wall of basalt, thirty feet 



