5. Metalliferous mincrals.—It contains many metal liferous minerals. They occur more frequently in veins than in beds ; but as the porphyry is seldom stratified, and as the surface of superposition is not often seen, it is difficult to determine to which of the two kinds of re positories they belong. The richest mines at present known, those of Mexico, are situated in enormous veins that traverse sienitic porphyry. The mines of Hungary, the most considerable on the continent of Europe, are situated in the same kind of porphyry ; and it would appear that the famous mines of Cyprus, so much ex tolled by the ancients, were also in porphyry. The numerous veins of lead, copper, and silver, worked at Giromagny, in the Vosges, are in a porphyry tract.
6. Formations.—Porphyry occurs in imbedded masses, beds, and veins of granite, gneiss, mica-slate, and clay slate.
7. Form of mountains.--They are often conical, some times like truncated cones, or appear dome-shaped.
8. Geographical distribution.—It occurs in the Shet land islands, in several of the Hebrides, and on the main land of Scotland, in Sutherland, Ross-shire, Inverness shire, Perthshire, Scc, In England it is met with in Cornwall, and in other districts. On the continent of Europe, it occurs in Sweden ; it forms a part of the Vosges, and rises in mountains in the granite district of Forez, also in France. It has not been met with in the Pyrenees, nor is it mentioned as occurring in the Alps of Switzerland, nor in the northern side of the grand chain of the Alps, but it occupies a considerable tract on the southern side, from the lake of Como to Carin thia and Carniola. It appears, although not very abun dantly, in Silesia, Saxony, and Thuringia, and forms extensive tracts in Hungary. It abounds in some dis tricts in Upper Egypt, Siberia, and in North and South America.
Uses.—It was formerly used extensively in ornamen tal architecture, and is still worked in considerable quantity as an ornamental stone, in Elfdal in Sweden, where there are considerable quarries of porphyry.
X. Quartz Rock.
Quartz fels.—Ifterner.
Quartz Rock.—Jameson.
Quartz en Roche.—Daubuisson.
Quartzite.—Brongniare and Bonnard.
1. Characters.—Quartz occurs not only as an essen tial constituent part of granite, gneiss, and mica-slate, and disseminated in beds and veins in these rocks, but also in mountain masses and mountains. Quartz-rock, properly so called, is generally of a white colour, and sometimes reddish or blueish. It has a granular struc ture ; the concretions vary from the smallest size, visible to the naked eye, to that of an egg, or even larger; or it is compact. It frequently contains grains of felspar, and
also scales of mica. When the felspar and mica increase in quantity, it passes into granite, or into gneiss, when only the mica into mica slate.
Structure.—It occurs either distinctly stratified, or massive, and without the stratified structure.
2. Metalliferous minerals.—It often contains dissemi nated iron pyrites, and occasionally lead-glance, copper pyrites, and blonde.
3. Form of mountains.—Mountains of quartz-rocks are often conical, sometimes even peaked, or they are crenated.
4. Gcognostic situation.—It occurs in beds and moun tain masses, in granite, gneiss, mica-slate, and clay slate, and indeed in a certain degree associated with most of the rocks of the primitive series.
Geographical distribution.—It abounds in many dis tricts in Scotland, as in the islands of Jura and Isla, the Shetland Islands ; on the mainland, in Sutherland, Caith ness, Inverness-shire, Argyleshire, &c.
Ubergansgebirge.— Werner.
Transition Hocks.—Jameson.
Terrains Intermediaires.—Daubuisson.
The rocks of the primitive class, as already remark ed, are distinguished in a general view by their highly crystalline structure, and want of petrefactions, or fossil organic remains. In some countries we observe resting upon them, and even alternating with them, a series of rocks, of which clay-slate is a predominating member, having less of the crystalline aspect, and which contains fossil organic remains. Werner considers this set of rocks as interposed between the grand series of primitive and secondary rocks ; and that, although it occasionally alternates, on the one hand, with the primitive, and on the other with some members of the secondary class, still its characters are so well marked, that he views it as a distinct class, to which he gave the name Transition, from its forming, as it were, the transition or passage from the primitive to the secondary rocks. Although some mineralogists have abandoned this view, and now arrange the transition rocks along with those of the pri mitive or secondary classes, we are still inclined to con sider them as deserving a separate place in the geognos tic system. It is true, that the transition rocks are but a continuation of the primitive, and, on a general view, might with propriety be considered as a portion of that series ; but their imbedded fossil organic remains, less crystalline aspect, and particular rocks, such as grey wacke, appear to characterise them, if not as a distinct class, yet as a separate group in the grand series of rock formations.