1844.] 299 



" This stone is found near Logrosan, a village in the jurisdiction 

 of Truxillo, in the province of Estremadura. It occurs, not in 

 veins, but forms entire hills. The houses and the walled en- 

 closures of the fields are built of it." 



On reaching Madrid, the authors were informed by the head of 

 the Department of Mines, that the mineral formed a vein in granite. 



The authors, after this introductory notice, then proceed to give 

 an account of their researches. 



Between the tertiary table-land of the two Castiles, and the 

 descent of the south-eastern escarpment of the Sierra Morena, as 

 you enter on the plains of Andalusia, a district of country inter- 

 venes, over a large portion of which a formation of clay-slate, with 

 occasional masses of quartzite, forms the fundamental rock. In 

 proceeding from Madrid, it was to the south of Talavera de la 

 Eeyna, at the village of Calzada de Oropesa, that this formation 

 first appeared. In the steep ravine through which the Tagus 

 flows near the broken bridge of Almaraz, dark blue slate appears 

 in vertical strata. Puerto di Miravete is the culminating point of 

 this formation, from which a vast table-land is seen intersected by 

 low flat-topped ridges, rising from three to four hundred feet 

 above the plain, and studded over with conical hills. The above 

 ridges, according to Le Play, are occasioned by beds of quartz > 

 and this statement agrees with the observations made by the 

 authors above Almaden. The quartzite is either compact, gra- 

 nular, or brecciated, or constitutes a fine-grained sandstone. The 

 conical hills consist of granite, which has forced its way through 

 the slate. Examples of this occur at Truxillo, and a league on- 

 wards on the road to Logrosan. Then slate re-appears, then 

 granite for the space of a mile ; then slate again ; which continues 

 to Logrosan. To the south of that village, granite again appears, 

 rising to the height of four or five hundred feet. With this ex- 

 ception, all the rocks around Logrosan, and thence as far as the 

 Monastery of Guadalupe, consist of the clay-slate and quartzite. 



The granite is much decomposed, and divides into blocks, which 

 strongly resemble Cyclopean walls. 



The follovsdng is a general representation of the respective situ- 

 ations of the slate and granite. 



a. Phosphorite beds of Logrosan. * 

 Base, seven Spanish leagues. 



* The slaty beds are grouped according to the following order : — 



1 . Dark blue, homogeneous, and excessively hard and compact fissile slate, 

 intersected by veins of quartz. This is the common building-stone at Logrosan. 



2. A soft and talcose slate. 



3. A micaceous slate. 



4. Alternating layers of talc and granular quartz. 



5. Brecciated and slaty beds. 



