76 Colonel Silvcrtop on the Lacustrine Basins of 



The sandstone is formed of small quartz grains, consolidated in- 

 to a mass by an arenaceous calcareous cement. Remains, appa- 

 rently of corals, are seen rarely imbedded in it ; but different 

 species of pectens, one of which is almost identical with a species 

 found in the London clay, are sufficiently abundant. It is of a 

 yellowish colour; and the fine conglomerates, as I have termed 

 them, into which it sometimes passes, only differ from it by the 

 size of the quartzose grains, and by the occasional interposition 

 of small fragments of schist, and of some other rocks. The se- 

 condary subjacent limestone, noticed in the little ravine on the 

 road to Loja, at the bottom of the hill of El Major de en Medio, 

 is not visible here, the whole of the escarpment, from its base to 

 its summit, being formed of the two varieties of rock just de- 

 scribed, the coral limestone occupying the superior part of the 

 section. It is in the interval between this and the bed of the 

 ravine, that alternations of the two members of this formation 

 are seen ; and it is observable, that near the contact of the lime- 

 stone with the sandstone strata, the former becomes gradually 

 less calcareous, and appears to be an admixture of quartzose par- 

 ticles, to pass gradually into the latter. In one instance I ob- 

 served a bed of loose unconsolidated quartzose sand, under up- 

 wards of one hundred feet of superior indurated strata. All the 

 strata, in their natural position, are horizontal ; but in the 

 higher part of the escarpments, large masses of them are now 

 and then seen variously inclined, probably by subsidence, and 

 contribute powerfully to enhance the picturesque scenery of this 

 fissure. 



The road from Alhama to Arenas (de Alliama), proceeds 

 for about one and a-half or two miles along the bed of this ra- 

 vine, and, after leaving it, enters upon a narrow cultivated flat, 

 contiguous to its right bank, and bordered, at a little distance on 

 the left-hand side, by a low escarpment of coral limestone, from 

 under which an insulated mass of older nummulite limestone 

 protrudes and intersects the line of road, rising but a few feet 

 above the general level of the ground. This is the second in- 

 stance clearly presented in this neighbourhood of such superpo- 

 sition. After subsequently ascending some hilly ground where 

 no rock appears, mica-slate of a reddish tinge, due to the de- 

 composition of its numerous imbedded garnets, projects above 



