235 



Spain, being the second of two letters addressed to Roderick Impey 

 Murchison, Esq., Sec. G.S., F.R.S. &c, by Col. Charles Silvertop, 

 F.G.S., was then read*. 



The basin of Alhama is situated about 50 miles to the south-west of 

 the basin of Baza, which was described in the former letter. It occupies 

 a large circular area, bounded on the south and east chiefly by the 

 primitive chain of the Sierra Nevada, and on the north-west and 

 south-west by ridges of nummulite-limestone. The greater diameter 

 of the basin, namely, between the village of Huerta de Santillana on 

 the north, and the ridge near Alhama on the south, is about 3G miles ; 

 and the smaller diameter, between the village of Escujar on the east, 

 and the town of Loja on the west, is about 30 miles. The principal 

 river traversing the basin is the Genii, which takes its rise in the 

 Sierra Nevada to the east of Granada ; and having received all the 

 minor streams which water the basin, it passes through a chasm in 

 the nummulite-limestone near Loja, and afterwards unites with the 

 Guadalquiver. 



The whole area of the basin, with the exception of an insulated 

 group of transition limestone rocks near Granada, is occupied by con- 

 glomerates, marl, gypsum, and limestones containing freshwater 

 shells. The conglomerates predominate to the north and east of 

 Granada, and form a high tract of waving hilly ground between that 

 city and the eastern part of the Sierra Nevada ; and the other depo- 

 sits prevail through the southern portion of the basin. The valley 

 through which the Genii flows is the lowest part of the district, and 

 is composed near Granada of a disintegrated conglomerate. 



The author gives a detailed account of the geological appearances 

 presented along the line of road from Granada to Alhama. The lower 

 strata consist of beds of gypsum alternating with strata of marl and 

 marly, micaceous sandstone. The gypsum is in general of the ordinary, 

 fibrous variety ; but near the village of Escuzar, alabaster of a beau- 

 tiful whiteness is quarried. In the bed of a rivulet passing by La Mala 

 a brine-spring issues, which yields from 18,000 to 24,000 fanegas of 

 salt yearly ; the fanega being equal to 251bs Spanish. The strata of 

 marl and gypsum are covered with a compact limestone, containing 

 casts of Paludinse ; and on this limestone rest irregular masses com- 

 posed almost entirely of comminuted shells of the genera Limnsea and 

 Planorbis. The fossils found in these limestones have been examined 

 by Mr. J. Sowerby, who has supplied the following list: — 



Planorbis rotundatus, found in the Paludina pusilla, of Deshayes. 



Isle of Wight. Paludina Desmarestii. 



Planorbis rotundatus vel planu- Paludina pyramidalis. 



latus. Ancylus. 



Planorbis, new species. Cypris. 



Bulimus pusillus, of Broard. Limnsea. 



The structure of the country around Alhama is explained by three 

 sections in the immediate vicinity of that village. One of these, ob- 

 served by following the horse-road from Alhama towards Loja, presents 



* For the first letter, see Phil. Mag. and Ann. of Phil., vol. vii. p. 453. 



