Phosphorite in Estrcmadura. 
413 
to a thickness of about three feet, consists of phosphate of hme in a state 
approaching to purity. 
The remainder is made up of alternate layers of hornstone and phos- 
phorite, disposed in an agatiform manner round certain nuclei of crystal- 
lization, the respective zones of the pure white variety being often 
separated, one from the other, by streaks or thin layers of the same, 
coloured with oxide of iron. 
In consequence of the mode in which the materials filling up the 
fissure have been deposited round particular centres, to which they are 
most forcibly attracted, void spaces frequently exist between the several 
concretions which constitute the entire mass. 
Where this has happened we often observe a mammillated structure, 
like that of certain chalcedonies, on the external surfaces of the mineral, 
■with which mode of formation, indeed, the structure of the internal 
layers appears on examination to accord. 
Crystals of quartz are occasionally found coating the external layers, 
and likewise lining the walls of the cavities in them. 
If we examine the particular structure of any one of those zones of 
phosphorite, which are wrapt round each other in the manner described, 
we shall find them often exhibiting a stellated arrangement, consisting 
of an assemblage of fibres or crystals radiating from a centre, as is the 
case in wavellite, for which mineral a small specimen of phosphorite 
might perhaps at first sight be mistaken. 
The external characters of the mineral itself have already been suffi- 
ciently described in works on mineralogy. 
The description given of it by Beudant defines in a few words, but 
with sufficient exactness, its peculiar structure, excepting that, misled 
by the erroneous notion as to its constituting " entire mountains," he 
represents as beds what are nothing more than layers or zones, running 
in a direction conformable to those of the phosphorite, and as veins, those 
which penetrate them transversely. 
" The apatite of EsLremadura," he says, " exists in fibrous, dendritic, 
stalactitic, testaceous deposits, either intermixed with quartzous beds, or 
intersected by veins of quartz, constituting entire hills near Truxillo, 
and employed as a building stone." 
The inaccuracy of the former part of this concluding passage we have 
already sufficiently adverted to ; the latter appears to have no better 
foundation than the use made of a few blocks of it close to the spot in 
which it occurs, in the construction of a wall which separates the road 
from an olive plantation contiguous. 
The deposit indeed is traversed by the road, and must at one time 
have formed an inconvenient rise across it. It had consequently been 
blasted or broken down, and as the fragments have been applied to 
patch the walls round about, we may account for the fable as to houses 
having been constructed of this material. At Logrosan the common 
building stone is the slate on which the town stands : it is dark blue, 
excessively hard and compact, with veins of quartz intersecting it. 
The phosphorite differs, of course, in composition in different parts of 
the deposit ; but no specimen which has been examined appears per- 
fectly free from foreign matter, there being, in every instance, traces of 
