54 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. 



The authors could not learn that, in the neighbourhood of Lo- 

 grosan, the slate contains any fossils, although, according to Le Play, 

 it contains near Almaden an abundance of Spirifer attenuatus 

 and of a Terebratula ; and, according to the miners of Almaden, 

 Trilobites had been found in it. These slates are referred to the 

 Silurian period. 



Between the granite and the slate there occurs a more crys- 

 talline rock, resembling mica slate, and said by Le Play to contain 

 chiastolite. The authors met with this rock between Almaden 

 and Cordova, near the granite of Viso. . 



Logrosan is a considerable village, about seven Spanish leagues 

 S. E. of Truxillo. In the neighbourhood of the village the surface 

 of the slate is undulated, the difference of level between the heights 

 and depressions being about fifty feet. It is in the clay-slate that 

 the Phosphorite occurs. See the section in the last page. 



It may be traced on the surface or immediately beneath the 

 soil, running in the direction* of the rocks themselves, that is, 

 from N. N. E. to S. S. W., for the distance of nearly two miles. 

 It terminates southwards not far from the base, and a little to the 

 east of the granite hill. The summit of the hill and its north- 

 eastern declivity consist of granite ; but the side nearest the phos- 

 phorite consists of clay-slate. At this point the phosphorite is 

 16 feet wide, and extends to an unknown depth. It has been 

 penetrated to the depth of only 10 feet. 



As was noticed by Bowles, the seam crosses the road leading 

 from Logrosan to Guadaloupe, and it forms an inconvenient rise 

 in the road where it crosses. To level this, the seam had been 

 broken down, and its fragments had been used to repair the 

 neighbouring walls, and in the construction of a fence that se- 

 parates the road from an olive plantation, and it seemed here to be 

 very little altered by exposure to the weather. The rock is a 

 compact clay-slate, of indistinct slaty cleavage, and is disposed in 

 beds inclining from the granite, but nearly vertical. The seam of 

 phosphorite is only 7 feet thick, and it is only the middle portion, 

 to the width of 3 feet, that is in a state of purity. The rest con- 

 sists of phosphorite, alternating with layers of hornstone, con- 

 taining iron and a trace of phosphate of lime. Other small 

 seams of phosphorite proceed obliquely from the main seam, and 

 penetrate the clay-slate to some distance. 



The mineral is disposed in zones, after the manner of agate, 

 round centres of crystallisation, each zone being an assemblage of 

 converging crystalline spiculae. Pure white zones of the mineral 

 are often separated, one from .the other, by their dark -brown 

 layers, tinged with oxide of iron. Between contiguous zones, 

 having different centres, void spaces often occur, and when this is 

 the case, the surface of the mineral next the cavity is mammillary. 

 Crystals of quartz also occasionally line these cavities. 



From the point where it crosses the road, the deposit was traced 



* Le Play speaks of the Phosphorite as intersecting the clay-slate. 



