46 Brigadier Silvertop^s Sketch of the Tertiary Formation 



are excavations or quarries of great magnitude, worked perpen- 

 dicularly, in a whitish-yellow argillaceous rock, intersected by 

 several little metallic veins (sulphurets of copper and iron). It is 

 probably a decomposed and altered feldspathic rock resting upon 

 trachyte. No authentic history exists upon the origin of these 

 excavations, but the neighbouring peasants attribute them to 

 the Romans, who are supposed to have extracted alum from the 

 rock. Soapstone, used in place of the article whose name it 

 bears by the peasants, blood-red jasper quart;?, chalcedony, 

 agates, and amethysts, are found in the immediate neighbourhood. 

 From this locality to the village of La Carbonera, a distance of 

 about twenty miles, the tertiary formation may be followed, — 

 in an irregular manner for the first four miles to the little sea- 

 port town of San Pedro, — but from the latter to La Carbonera 

 almost continuously. Its greatest breadth along this line of 

 coast appears to be about five miles. The deposit is variously 

 composed of argillaceous marl, sandy loom, calcareous free- 

 stone, hard compacted quartzose freestone, loosely aggregated 

 sandstone, and zoophytical limestone. In the immediate vi- 

 cinity of San Pedro, a thick bed or deposit of horizontal strata 

 of yellowish zoophytical limestone, has once formed an elevated 

 and extensive table-formed tract, capping or overlying the rocks 

 of the volcanic ridge which passes by this little sea-port, in its 

 range along the coast from the Cabo de Gata to the village of 

 La Carbonera. This tract has been partially denudated, as 

 seen in section 8. ; and the subjacent trachyte rock is exposed 

 to view in a consequent hollow or depression, that separates a 

 high tertiary hill, which extends for about three miles along the 

 coast immediately to the north of San Pedro, from a minor 

 eminence of the same deposit more retired from the store. 



The old castle and the little hamlet of San Pedro are built 

 upon these tertiary beds, in a deep ravine or denudation be- 

 tween the high hill just alluded to, and another of the same 

 order to the south of the village, the road or path to which from 

 Roalquilar is carried along a slope of volcanic tuiT, at the base 

 of a high escarpment of the laltej-, to the Mediterranean Sea ; See 

 Sect. 9. PI- III. This section presents one of many instances 

 observed in the vicinity of Sad Pedro, of the immediate super- 

 position of the tertiary beds to volcanic tuff, indicating that an 



