Very little work seems to have been done on the petrology of the rarer constituents of the older Palaeozoic sediments and their associated igneous rocks, partly no doubt on account of the extra amount of work entailed in crushing these extremely hard compact rocks, and partly owing to the comparative paucity of the “heavy” residues obtained, which give small reward for the energy expended. Such work, however, is highly important as indicating further sources of supply, to which workers on the petrology of the newer rocks may look, and it may be here emphasized that Wales has unfortunately suffered neglect in this respect. It is to help remedy this defect that the present paper is put forward.

The Province of Huelva, famed for its mines from the times of the Ancients, lies in the south-western portion of Spain, in the upland between the basins of the Guadalquivir and Guadiana. To the north are the serrated ridges of the Sierra Morena and to the south the fertile plains of Huelva and Sevilla. This upland, now in a mature stage of erosion, forms the old coastal plain. It is composed of Palaeozoic rocks, chiefly Carboniferous, which lie unconformably on the pre-Cambrian gneisses and schists of the Sierra Morena. Miocene limestone and Pleistocene Drift in places obscure the Palaeozoic rocks. The Carboniferous rocks are principally slates and grits, the former being by far the more abundant. In what is usually regarded as Hercynian times and following great tectonic movements, the Carboniferous rocks were invaded by granites, porphyries, diorites, porphyrites, and diabase. The mineralization was associated with these intrusives and principally with the porphyries. Finally the area was reduced to a peneplain by erosion.

Jointing in rocks is a key to the strains and movements they have undergone. In rocks which have had a simple history we expect to find a simple arrangement of joint planes: in old rocks we expect to find evidence of several series of tectonic movements and hence complicated jointing. For example, in the Cambrian quartzite south of Dublin the joint planes appear to run in all directions with no obviously dominant trend. But if say fifty readings taken haphazard in one of the quarries are carefully analysed it becomes evident that the joints tend to lie in certain directions, which, if a graphical method is used, appear as “peaks” on the diagram. On examining other exposures it becomes clear that some of these peaks are of regional significance, for they recur on the diagrams for many or all of the quartzite quarries. The diagrams also show, what is not evident in the field, that in certain directions jointing is almost entirely absent.