A. M. Finlayson—Petrology of Huelva, Spain. 225 
show complete sericitization of felspar, leaving the quartz unaltered. 
Included portions of the ground-mass in quartz phenocrysts are also 
commonly preserved from the alteration which has affected the rest 
of the rock. Along the margins of the intrusions dynamic effects are 
particularly marked. Here the porphyries have frequently been 
converted to highly cleaved rocks with a greasy lustre. Under the 
microscope these show the same mineralogical changes as the more 
massive porphyries, carried to a greater extreme, while the effect 
of pressure has resulted in perfect schistosity, marked by folia of 
felted sericite (Fig. 3), which enclose granulated crystals of quartz 
and occasional aggregates of sericite and epidote representing altered 
felspars. As indicating the origin of these foliated types, it is 
significant that they correspond, in the nature of their phenocrysts, in 
each case with the adjoining massive porphyries. The coarser and 
more porphyritic rocks are accompanied by brecciated rather than 
foliated modifications, in which the crystals are arranged in a broken 
confused aggregate, which is, in the hand-specimen, with difficulty 
_ distinguishable from a pyroclastic rock. The absence of foreign 
constituents, however, and the remarkable uniformity of these cleaved 
and crushed types, as well as their microscopic structure and relations 
to the less altered porphyries, indicate that their present structure is 
the result of pressure and movement. 
The age and relations of this series of rocks have been considerably 
discussed. Most of the geologists who have examined them, including 
J. H. Collins,! Gonzalo y Tarin,? L. de Launay,*® and J. H. L. Vogt,* 
have regarded them as intrusive into the slates. The highly cleaved 
marginal phases were considered by Collins to be the result of pressure 
during solidification, by Tarin as effects of contact-metamorphism, and 
by Vogt as due to subsequent pressure and shearing. On the other 
hand, F. Klockmann has maintained, after considerable investigation, 
that the porphyries are subaqueous lava-flows contemporaneous with 
the deposition of the slates, and that the foliated and cleaved rocks 
associated with them are tuffs and ash-beds, which have been 
subsequently sheared.° 
The present writer’s conclusions, after the examination of a large 
series of exposures and of collected specimens from various parts of 
the district, are in agreement with those of Vogt and opposed to 
the views of Klockmann. The chief reasons for concluding that the 
porphyries are intrusive, and that the cleaved varieties are due to 
later pressure and movement, are as follows :— 
(1) The occurrence of micrographic intergrowths and of corroded 
1 «* Geology of the Rio Tinto Mines’’: Q.J.G.S8., 1885, vol. xli, p. 245. 
2 “Descripcion fisica, geologica, y minera de la provincia de Huelva’’: Mem. 
Com. Map. Geol. Esp., Madrid, 1886, vol. i. 
3 <* Mémoire sur |’ industrie du cuivre dans la région d’Huelya’’: Ann. d. Mines, 
1889, ser. virt, vol. xvi, p. 407. 
4 <Das Huelva-Kiesfeld in siid- Spanien und dem angrenzenden Theile yon 
Portugal’’: Zeits. prakt. Geol., 1899, vol. vii, p. 241. 
> F. Klockmann, ‘‘ Uber die lpeesc ee Natur der Kiesvorkommen des stidlichen 
Spaniens und Portugals” : Sitzungsber. d. preuss. Akad. d. Wissensch., Berlin, 
1894, vol. xlvi, p. 1173; and ‘* Ueber das auftreten und die entstehung ‘der siid- 
spanischen Kieslagerstatten”? : Zeits. prakt. Geol., 1902, vol. x, p. 113. 
DECADE V.—VOL. VII.—NO. V. 15 
