228 A. M. Finlayson—Petrology of Huelva, Spain. 
movements, the lodes being formed in the zones of sheared or 
shattered rock. 
The district affords a good illustration of Aes features. In the first 
place, the development, by magmatic differentiation, of the series of 
igneous rocks has been closely “connected with the earth-movements 
which have given the area its present structure. The igneous rocks 
and the tectonic structure are coextensive, and the. magmatic 
differentiation has doubtless been attendant on the sub-crustal stresses 
of the Hercynian epoch. In the second place, the relation of the ore- 
deposits to the igneous rocks is, broadly speaking, a genetic one. 
The defined petrographic province, limited to a certain area, is 
accompanied by the equally defined group of pyritic ore-bodies, limited 
to practically the same area. In other words, there is here a 
metallogenetic province accompanied by a corresponding petrographic 
province. The association indicates the close dependence of ore- 
formation on earth-stresses and magmatic processes. While there 
is nothing to show that the lodes are genetically related to one 
particular group of igneous rocks, there is strong reason for believing 
that the concentration of the sulphides has been primarily a magmatic 
process, intimately bound up with the progressive differentiation which 
resulted in the igneous rocks now exposed. ‘The ore-deposits, in 
short, represent the final product of the magmatic processes, just as 
the fault-zones in which the lodes occur were the last result of 
the earth-movements with which these magmatic processes were 
involved. 
' he Huelva copper-field is “closely paralleled in almost every 
respect by the Avoca district in co. Wicklow.! Here a belt of 
Palsozoic slates is succeeded to the west by the Leinster granite 
massif, and the slates are intruded by a series of abundant sills and 
dykes of both acid and basic rocks, in part contemporaneous with and 
in part later than the granite. These intrusions have been intensely 
altered by subsequent pressure and movement,” and, as in the Huelva 
district, there is a general parallelism in the strike of the slates and in 
the alignment of both the granite and the abundant smaller intrusions. 
Finally, there occurs in the slates, and on the same general trend, 
a belt of pyritic lodes, sometimes enclosed in slate and sometimes 
adjoining sills of felsite or greenstone, but always in zones of 
‘shearing and:crushing. Both structurally and mineralogically the 
ore-bodies of Avoca and those of Huelva show exactly the. same 
features. Specimens of ore from the two fields are indistinguishable 
under the microscope. It is clear that in both districts similar 
agencies have been at work, with the production of exactly similar 
results. 
EXPLANATION OF PLATE XVIII. 
Fic. 1. Rhyolite-porphyry, Rio Tinto. The specimen contains quartz phenocrysts 
and abundant smaller grains of quartz, embedded in a felsitic or crypto- 
crystalline matrix, which has been largely converted to ‘sericite. The 
rock is impregnated with pyrite and somewhat sheared. x 36. 
1 Explan. Sheets 138 and 139, Geol. Surv. Ireland, 1888. 
* Sum. Prog. for 1900, Geol. Surv. Great Britain, p. 51. 
