224 A. MW. Finlayson—Petrology of Huelva, Spain. 
Petrologically, the most common type is a hornblende-granite, but 
therocks vary from muscovite- and biotite-granites, through hornblende- 
granite and syenites, to granodiorites. Monzonite and tonalite also 
occur among them, considerable variation being shown in individual 
bosses. ; 
2. Porphyries.—These rocks are very abundant both in the 
northern district of older rocks and in the mineral belt. In the 
former area they generally occur as marginal phases of the more 
deep-seated granite members, and also as later dykes intrusive into 
these. In the mining-field they frequently occur alongside the lodes, 
and owing to their profusion lodes are seldom found at a great 
distance from them. 
The rocks show considerable variety, ranging, like the granitic rocks, 
from acid to intermediate types, with some alkaline phases. The most 
common type. is probably a quartz-porphyry or rhyolite-porphyry, 
which is well developed at Rio Tinto (Pl. XVIII, Fig. 1). This rock 
contains abundant coarse phenocrysts of quartz, often corroded, and 
with inclusions of the felsitic ground-mass. The felspars are chiefly 
orthoclase and albite, but do not occur as phenocrysts in the more 
acid members. Subordinate muscovite and biotite are generally 
represented by chlorite and other alteration products. The ground- 
mass is felsitic or cryptocrystalline, and frequently contains abundant 
coarser grains of quartz. At times micropegmatites of quartz and 
albite occur, and the rocks become granophyric quartz-porphyries. 
With an increase in albite and microcline, quartz-keratophyres occur 
as local phases of the porphyries at Rio Tinto and elsewhere (Pl. XVIII, 
Fig. 2). A coarser-grained eurite, containing orthoclase and quartz, was 
observed near El Cerro, on the raiiway-line from Huelva to Zafra. 
Larger masses, such as occur in the neighbourhood of the Sotiel Mines, 
are typically granite-porphyries. 
In the less acid members quartz phenocrysts disappear, the rocks as 
a rule are coarser, and include orthophyre or trachyte-porphyry, 
syenite-porphyry, and monzonite-porphyry. Such rocks occur north 
of the Tharsis Mine, at Sotiel, and elsewhere. Micrographic inter- 
growths are absent, and green hornblende is sometimes present, while 
the felspars are chiefly oligoclase and andesine. 
As a rule the porphyries are found as sills and sheets, while the 
larger masses form bosses. In all cases, however, like the granites, 
they are disposed in belts parallel to the trend of the older sedimentary 
rocks. Owing partly to intense dynamic metamorphism of later 
date, contact effects are not well marked adjoining them, and 
mineralogical alteration of the slates can seldom be detected. The 
most usual evidences of intrusion are the presence of bands of 
porcellanite along the margins of the porphyries, and the occurrence 
of inclusions of baked slate. 
Since their intrusion this series of rocks has been subjected to 
profound dynamic metamorphism. This is marked by straining and 
granulation of the quartzes, by crushing and complete sericitization of 
the felspars, and by the conversion of the ground-mass into an aggregate 
of quartz and finely divided sericite. Colourless epidote is often 
abundant as an alteration product of the felspars. Micropegmatites 
