108 Mr. D, Sharpe on the Geology of 



lowing the descending order, both in the general arrangement of the subject and 

 in the tables of beds and description of sections ; and then the igneous rocks, with 

 the disturbances which the sedimentary deposits have undergone. A few remarks 

 on the great earthquake of 1755 are given as an appendix. 



PART I. — Aqueous Deposits. 

 Tertiary Formations. 



The tertiary beds appear to be entirely of marine origin, and they may be divided 

 into three formations : 



1. Upper Tertiary Sand. 



2. Almada Beds ; consisting of calcareous sands, sandy limestones, marls and clays. They 



may be best observed in the cliffs on the south bank of the Tagus, near the village of 

 Almada. 



3. Lower Tertiary Conglomerate. 



These deposits occupy a basin, of which only a part is included in the region 

 described in this paper. Their greatest extent is from the mouth of the Tagus to 

 Abrantes, a distance of about eighty EngHsh miles ; and from Lisbon to Alcacer 

 do Sal, about fifty miles. The Tagus runs through this basin from Abrantes to 

 the sea, keeping close to its northern boundary, so that a very small portion of it 

 lies north of the river. 



1. Upper Tertiary Sand. (PI. XIV. and PL XV., Sect. 1, 9, 11 to 13.) 



This formation consists of about 100 feet of fine grey quartzose sand and 150 

 feet of coarse, quartzose, ferruginous sand and gravel. 



On the south of the Tagus it covers nearly the whole of the tertiary district. 

 Its beds are quite horizontal, except where they rest upon the inclined strata of 

 the Almada formation towards the edges of the basin, and even there, the dip never 

 exceeds ten degrees : I did not notice a single instance of their having been dis- 

 located. 



Baron d'Eschwege pointed out to me a very remarkable rock on the banks of 

 the Sado, half a mile above St. Ubes, called the ' Pedra Furada ' or perforated 

 rock, which belongs to this formation, and consists of irregular, perpendicular 

 tubes varying from half an inch to a foot in diameter, of a very ferruginous sand- 

 stone, united by horizontal layers of the same substance : the cement of the sand- 

 stone is phosphate of iron ; the centres of the tubes and the spaces between them 

 are filled with ochreous sand, which passes into the usual sand of the neighbour- 

 hood. The rock is about twenty feet in height and width ; its extent inwards 

 from the river is concealed by the overlying sand. 



I did not explore the whole of the boundary of the upper tertiary sand, which 

 includes considerably more than 1000 square miles. North of the Tagus it is found 



