Phosphorite in Estremadtira. 
411 
down the surface of the slates contiguous, and thus to maintain more 
nearly its original level, just as in the Tyrol the incoherent materials, 
which have resulted from the detritus of older rocks, remain standing 
up to the height which they possessed when first deposited, wherever 
there happened to lie at the top of them a mass of stone large enough 
to shelter the parts underneath from the action of rain. 
The beds of quartzite, which occur subordinate to, and interstratified 
with, the slate, present many varieties of structure, being sometimes 
compact, sometimes granular, possessing occasionally a brecciated cha- 
racter, and in other places hardly distinguishable from a fine-grained 
sand-stone. 
The clay-slate itself, which constitutes the prevailing rock about 
Logrosan, is sometimes a dark blue homogeneous fissile schist ; some- 
times soft and talcose : it at other times contains scales of mica dissemi- 
nated through it; and at others consists of alternatmg layers of compact 
felspar and of talc. Brecciated varieties also occur, which, but for their 
fissile character, we might refer to quartz rock ; but, in fact, there is no 
decided line of demarcation between the two. 
The most important question relative to this and its concomitant rocks 
relates to their age, and here the evidence is chiefly negative. 
During our excursions, about Logrosan, in the mountains of Guada- 
lupe, and elsewhere, we met with no vestige of shells, or of other kinds 
of organic remains, nor could we learn that any such had been dis- 
covered in them. 
Le Play, however, states that in the neighbourhood of Almaden slate 
rocks occur, in which shells are abundant ; and he specifies the Spirifer 
attenuatus of Sowerby, and a Terebratula with small sides and large 
convolutions, as having been there met with. 
We were also told, during our stay at Almaden, by some of the intel- 
ligent miners connected with that escablishment, that Trilobites had been 
found in the same neighbourhood; so that no doubt need be enter- 
tained that rocks occur thereabouts which we may refer, as indeed 
the Spanish authorities at Madrid are disposed to do, to the Silurian 
epoch. 
But whether the rocks surrounding Logrosan are precisely of the 
same age as about Almaden, or whether, as Le Play represents, those 
near the latter place consist of a more modern fossiliferous formation 
superimposed upon an ancient one in which fossils have not yet been 
discovered, must remain undecided, until a sufficiently extended and 
minute survey has been made of the entire province, either to detect the 
presence of petrifactions in them, or to justify us in pronouncing upon 
their absence. 
In many cases, intervening between the granite and clay-slates, occurs 
a formation of a more crystalline character, which we found to assume 
the appearance of mica slate between Almaden and Cordova, near its 
contact with the granite of Viso, and which M. Le Play reports to be in 
other cases marked by the presence of crystals of chiastoiite (schist 
maclifere). 
It is in this clay-slate formation, which roundabout Logrosan seems 
remarkably compact, that the deposit of phosphoric occurs. 
