54 DEPOSITS OF PHOSPHATE OF LIME. [bull. 40. 
such as Spirifer attenuatus and trilobites. Consequently the slate 
formation of Logrosan and Caceres has been referred by some French 
and Spanish geologists to the Silurian formation. Le Play 1 refers 
it to an older formation than the fossiliferous slates of the neighbor- 
hood of Almaden. The conical peaks rising up from this formation 
are granitic intrusions. They are very feld spathic and often much 
weathered. In this latter respect they differ from the long, flat 
ridges spoken of above. These are beds of quartzite interstratified 
with the country rock, which lias a quite regular, almost vertical 
dip. The quartzite is sometimes very compact and homogeneous 
and at others it is granular and often resembles a sandstone. It has 
resisted the erosive action, which has worn down the more easily at- 
tacked parts of the slate formation and now stands out in bold, angular 
ridges. The section (Fig. 24) from Truxillo to Logrosan, a distance of 
seven Spanish leagues, will show the general character of the country. 
Besides the rocks already mentioned, large veins of dark limestone 
are occasionally found cutting through the slate formations. 
Fig. 24. Section from Truxillo to Logrosan, Spain, after Daubeny and Widdrington. A, granite; 
B, slate ; C, phosphorite. 
The phosphorite occurs in true veins and in pockets. Occasionally 
it occurs as a vein at the line of junction of the granite with the country 
slate. It is of a variable character, occurring sometimes in an amor- 
phous and compact form, at others in a fibrous or concretionary state, 
often inclosing pebbles of white or ferruginous quartz. It varies in 
color from white and yellow to a rich, jasper-like red. It is often cov- 
ered with dendrites of manganese, and occasionally agate-like varieties 
are found in which the phosphate is interstratified with bands of lilac 
amethyst. The palmated variety is generally the purest and the most 
abundant. It has a hardness of 5.5, and a specific gravity of ^12. 
When heated in a darkened room it gives off a bright phosphorescent 
light. 2 
At Logrosan there are six principal deposits of phosphorite. They 
are known as Costanaza, Jungal, Castillon, Angustias, Terrenos Co 
lorados, and La Cambre Bojera. The Costanaza vein is by far the 
largest phosphorite vein known in all Spain, and perhaps in the world 
(Shepard). 3 It extends for about two and a half miles from the foot of 
Mt. Boyales, on the north, in a southeasterly direction past Mt. Cristo- 
bal- The vein dips at an angle of 60° to 90° toward the east (Garza and 
1 Aunales des mines, 1836. 
2 Sur la phosphorite de Logrosan, Estremadura, Messrs. Naranjo y Garza and Lino 
Penuelas : Bull. Soc. gdologique France, 1860, vol. 17, p. 167. 
3 Foreign Phosphates, 1879, p. 25. 
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