1835.] Estramadur a and the North of Andalusia. 347 



uniform nature of the causes which have given origin to 

 the different formations. They lie over the mica slate in 

 the following order : clay slate and talc slate, alternating 

 with greywacke, and always with thick beds of quartz. 

 Limestone also is common on the heights between Llerena 

 and Guadalcanal, apparently belonging to the same series 

 of rocks. The mountain chain extending to the south of 

 Almaden consists principally of layers of quartz, without 

 any trace of fossils ; but at the lower grounds, where we 

 find sandstone and phyllades, passing into psammites, 

 remains of animals are frequently encountered, similar to 

 those of the secondary transition rocks of Britain. 



In particular, a terebratula, with large whorls, and a 

 fossil corresponding exactly with the spirifer attenuatus of 

 Sowerby, have been met with, and similar remains have 

 been observed at St. Eufemia and Espiel. Hence, it 

 appears that there have been two periods in the transition 

 formation of Almaden, but their proper discrimination 

 would require a very attentive examination. 



Clay slate alternating with talc slate, constitutes the 

 basis of the transition formation of Estramadura. The first 

 has a fine grain, slightly sonorous, with a beautiful blue 

 slate colour, and is sometimes separable into very thin 

 plates. The latter varies a good deal in the relative pro- 

 portion of its constituents, and passes often into phyllades, 

 a rock which contains much mica, agglutinated by a sandy 

 or earthy basis, and frequently resembling the psammites 

 of the variegated sandstone formation. Greywacke, how- 

 ever, is the predominant transition rock. Between the 

 Sierra d' Almaden and the Rio Guadalamez, it is very uni- 

 form in its structure, and is the only rock met with between 

 the granite of Albuquerque and Malpartida. It abounds, 

 although mixed with phyllades, in the neighbourhood of 

 Talarrubias, Orellana, Espiritus Santi Cabeza del Buey. It 

 is generally characterized by consisting of compact, hard 

 and fine grains, with a gray colour. 



The quartz rocks do not exist to such an extent as in 

 Scotland, but are merely found in thick beds subordinate 

 to the transition rocks. In the hills between Espiritus 

 Santo and Almadan, the quartz is very compact, resembling 

 eurite, and frequently passes into sandstone. M. Le Play 



