the Neighbourhood of Lisbon. 109 



only in low lands skirting the river near Santarem and Cartaxo ; but it covers as 

 much of the country between the Tagus, the Sado, and the sea, as would be cutoff 

 by a line drawn from Abrantes to Alcacer do Sal, with the exception of two ridges of 

 hills of older formations, which will be best seen by reference to the annexed map. 

 It usually rests upon the Almada Taeds ; but between the lagoon of Albufeira and 

 Cape Espichel it overlaps that formation, and rests upon the red sandstone. Its 

 greatest elevation above the sea is about 1 50 feet. 



I observed no organic remains in this formation. 



The sands of the upper division present barren, dreary heaths, which hardly ad- 

 mit of any cultivation ; but the fir thrives in the ferruginous sands of the lower 

 division, which are cultivated here and there, though to little advantage. The only 

 interest attached to this most barren formation arises from its mineral contents, 

 which are richer than might be expected from so modern a deposit. 



Mineral Contents. — A mine of quicksilver was discovered in the beginning of the last century at Coina, 

 by Manoel da Cruz Santiago, an individual celebrated in the annals of Portuguese mining. He had 

 observed that a spring of water in the centre of the village was strongly impregnated with mercury. No 

 account of this mine has been published, and I could only learn that it was worked profitably at a very 

 slight depth until 1801, when it was abandoned, all the quicksilver having been extracted, which their 

 method of working enabled the miners to reach. From the position of the spot, I have no doubt that it 

 was worked in the lower beds of this formation. 



From a very early period the sands of the Tagus have been stated to contain gold-dust. It appears 

 to be disseminated in very small quantities through the whole of the lower division of this formation, as 

 it has been found in several places where the heavier particles of the sands of that division are sepa- 

 rated by the action of rain or of the streams of water. The only spots where workings for gold-dust 

 have been carried on regularly to any extent, are on the coast near Adi9a, about ten miles south of the 

 mouth of the Tagus. The beach lies upon a bed of blue marl belonging to the Almada formation, but 

 the cliffs consist of the ferruginous sands. The numerous springs which flow through them wash down 

 large quantities of sand, and the waves give them another washing upon the beach ; the workmen dig 

 up this sand and submit it to a further process of washing, and the residue is treated with mercury to 

 separate the gold. 



Baron d'Eschwege, who superintended these works for some years, has given an account of them in 

 the eleventh volume of the Memoirs of the Academy of Sciences of Lisbon, and he has appended to it 

 a statement of the result of the operations. He mentions that works were carried on at Adi^a in the 

 fifteenth century as long as they were profitable, and that they were recommenced in ISl* and conti- 

 nued till 1826, when they were again found a losing concern. During these thirteen years 373 marks 

 of gold were collected, varying in fineness from twenty-two to twenty-three carats. It appears that a 

 remunerating quantity of gold is collected only after the natural washings by the springs and the sea 

 have gone on for a considerable number of years; and therefore, another long interval must elapse before 

 the washings can be again profitable. 



2. Almada Beds. (PI. XIV. and PI. XV., Sect. 1, 3, 6 to 9, and 1 1 to 13.) 



This is the most interesting formation of the tertiary series, in consequence of 

 the variety of its rocks and the abundance of its organic remains. It may be 



