124 Mr, D. Sharpe on the Geology of 



extremity of which it is laid open by the St. Ubes' road. The strike is there nearly 

 E. and W., and the dip N. at angles varying from 30° to 50°. The eastern end of 

 the Serra de San Luiz consists of the same rock, also dipping N. at about 50°. The 

 conglomerate forms all the higher parts of the Serra de Vizo, and the coast to the 

 beginning of the Serra de Arrabida. Along this line there is great variation in 

 the dip of the beds : at the east end of the Serra de Vizo, the inclination is south 

 about 50° ; more to the west it is in some places vertical, and at others 50° to the 

 north ; while at the Torre de Outao, at the foot of the Serra de Arrabida, it is 

 N.E. about 70°. 



The lowest beds of the formation are not seen in this neighbourhood, nor is the 

 rock upon which they rest. 



A conglomerate of similar character is largely developed upon the banks of the 

 Vouga, and overlies mica-slate a little to the south of Oporto. 



PART II. — Igneous Rocks. 



It remains for me to describe the igneous rocks which occur near Lisbon, and to 

 point out the alterations which they appear to have caused in the position of the 

 aqueous formations. 



Basalt. (PL XIV. and PI. XV. Sect. 1 to 3, 6 to 8, 10 and 12.) 



The principal deposit of this rock forms one of the most important features in 

 the geology of the district ; but it extends so irregularly on the west and north of 

 Lisbon, that its range can be understood only by referring to the map. Its extreme 

 length, from S^^ Catarina on the Tagus to its northern boundary near Bucellas, is 

 about twenty miles, but its width varies greatly. Besides this immense sheet of 

 basalt, many of the hills around Oeiras, near the mouth of the Tagus, are capped 

 by masses of the same rock, which have evidently been separated from it by the 

 excavation of the intervening valleys. The extent of country occupied by this 

 deposit cannot be less than eighty square miles, the whole of which appears to have 

 been covered by one eruption ; and if, as is probable, the basalt reaches far under 

 the tertiary beds, the amount of the eruption must have been greater. 



The basalt varies much in character : in some places it is a hard massive rock 

 without any marked joints ; elsewhere it occurs in large blocks approaching in 

 form to regular polygons ; occasionally it is columnar, and often it is merely 

 an indurated black clay, with an unequal schistose cleavage, splitting into very 

 irregular rhombs. It is usually compact, but frequently amygdaloidal or slightly 

 crystalline; and these varieties commonly pass into each other. The best columns 



