226 A. M. Finlayson— Petrology of Huelva, Spain. 
quartz: phenocrysts with inclusions of ground-mass indicates hypa- 
byssal. rather than volcanic rocks. 
(2) In a single exposure of porphyry the texture is frequently 
found to vary from coarse to fine in passing from the centre to the 
edge of the mass. 
(3) Intrusion is indicated by poreellanous 1 margins and by the 
presence of inclusions of slate, which have been caught up in the 
porphyries. 
(4) In some cases an examination of the line of contact showed 
that it crossed the bedding of the slates, and that a later cleavage 
had been developed in the slates parallel to this line of contact. 
(5). The occurrence of fine-grained porphyries as marginal phases 
of the granites in places is opposed to the view that they are layva- 
flows, as is also the occurrence of exactly similar porphyries in rocks 
of Silurian and Cambrian age in the north of the province. 
(6) The close correspondence of the cleaved varieties with the 
associated massive porphyries in each case, the great uniformity of 
the cleaved types, and the absence in them of foreign constituents, - 
all go to show that they are not of pyroclastic origin. 
It is therefore concluded that the porphyries are intrusive into 
the slates, and that since their intrusion they have been affected by 
intense pressure, which has given rise to shearing along the lines 
of contact with the slates, which were naturally lines of weakness. 
To this shearing has been due the development of the cleaved and 
foliated varicties of porphyry. If this is the correct interpretation— 
and all the evidence gathered in the present work goes to show 
that it is—then the porphyries must, from their petrological analogy 
to and frequent association with the granitic rocks, belong to the 
same stage of igneous activity as these, but to a less deep-seated 
phase of consolidation. They were therefore intruded along with 
the plutonic masses during the disturbances of the Hercynian epoch. 
At the same time, the intrusions were probably spread over a con- 
siderable interval, as dykes of granophyre and felsite occur in places 
intrusive into the granites. 
8. Basie Intrusions—These have a similar distribution to the 
porphyries, but are less abundant. In the rocks of the mining field 
they occur generally as sills and occasionally as bosses. They 
include diabases, augite-porphyrites, dolerites, and augite-diorites. 
Augite is very abundant and generally ophitic in habit. The rocks 
have, as a rule, a high titanium content, marked by ilmenite, by 
pleochroic augite, and by abundant perovskite. ‘The felspars vary 
from andesine to anorthite, brown hornblende is occasionally present, 
and olivine very rarely. The adjoiing contact-altered slates show 
the development of, first, chlorite, and finally, abundant yellowish- 
brown epidote. 
That this basic group, whose members appear from their generally 
close mineralogical resemblance to be all of the same age, is the 
youngest of the series of igneous rocks in the district, is indicated by 
the fact that they are found in places intrusive into both granites and 
porphyries, while they have been also intruded into the Triassic rocks 
at Ayamonte. Further, they are unaffected by the cleavage and 
