Elaeolite- and leucite- (syenite) porphyries form apophyses and dikes around nepheline- and leucite-syenite intrusions. The former contain porphyritic nepheline which is often weathered to soft, finely crystalline aggregates of white mica and other secondary products, as in the well-known liebenerite-porphyry of Tirol and gieseckite-porphyry of Greenland. The felspars of these rocks are albite, orthoclase and anorthoclase and they often contain soda-augite and amphiboles. Elaeolite-porphyries occur along with nepheline-syenites in such districts as the Serra de Monchique, south Norway, Kola, Montreal. Allied to them are the tinguaites (so called from the Serra de Tingua, Brazil), which are pale green rocks with abundant alkali felspar, nephe line, needles of green aegirine, and sometimes biotite and can crinite. As a rule, however, these are not porphyritic. Grorudites are quartz-tinguaites free from nepheline, and solvsbergites are tinguaitic rocks in which neither quartz nor nepheline occur. The two last have been described from the Oslo district in Norway, but tinguaites are known with nepheline-syenites in many parts of the world, e.g., Norway, Brazil, Portugal, Canada, Sweden, Greenland.
The following analyses of porphyries of different types show the chemical composition of a few selected examples :— Porphyrites.—The porphyrites as above mentioned are in trusive or hypabyssal rocks of porphyritic texture, with pheno crysts of plagioclase felspar and hornblende, biotite or augite (sometimes also quartz) in a fine ground-mass. The name has not always been used in this sense, but formerly signified rather decomposed andesitic and basaltic lavas of Carboniferous age and older. Both the red and green porphyry of the ancients are more properly classified in this group than with the granite-porphyries, as their dominant felspar is plagioclase and they contain little or no primary quartz. Porphyrites occur as dikes which accom
pany masses of diorite, and are often called diorite-porphyrites; they differ from diorites in few respects except their porphyritic structure. The phenocrysts are plagioclase, often much zoned, with central kernels of bytownite or labradorite and margins of oligoclase or even orthoclase. In a special group there are cor roded blebs of porphyritic quartz: these are called quartz-por phyrites, and are distinguished from the granite-porphyries by the scarcity or absence of orthoclase. The hornblende of the por phyrites is often green but sometimes brown, resembling that of the lamprophyres, a group from which the porphyrites are sepa rated by their containing phenocrysts of felspar, which do not occur in normal lamprophyres. Augite, when present, is nearly always pale green; it is not so abundant as hornblende. Dark brown biotite is very common in large hexagonal plates. The ground-mass is usually a crystalline aggregate of granular felspar in which plagioclase dominates, though orthoclase is rarely ab sent. Diorite-porphyrites have almost as wide a distribution as granite-porphyries, and occur in all parts of the world where in trusions of granite and diorite have been injected; they are in fact among the commonest hypabyssal rocks.
To gabbros and norites certain types of porphyrite correspond which have the same mineral and chemical composition as the parent rocks but with a porphyritic instead of granitic structure. Norite-porphyrites have porphyritic plagioclase (labradorite usually) with hypersthene or bronzite, often altered to bastite. They accompany norite masses in Nahe (Prussia) and Tirol.