A feature of these rocks is the presence of large foreign crys tals or xenocrysts of felspar and of quartz. Their forms are rounded, indicating partial resorption by the solvent action of the lamprophyric magma ; and the quartz may be surrounded by corrosion borders of minerals such as augite and hornblende, produced where the magma is attacking the crystal. These crys tals are of doubtful origin ; they are often of considerable size and may be conspicuous in hand-specimens of the rocks. It is sup posed that they did not crystallize in the lamprophyre dike but in some way were caught up by it. Other enclosures, more cer tainly of foreign origin, are often seen, such as quartzite, schists, garnetiferous rocks, granite, etc. These may be baked and altered or in other cases partly dissolved. Cordierite may be formed either in the enclosure or in the lamprophyre, where it takes the shape of hexagonal prisms which in polarized light break up into six sectors, triangular in shape, diverging from the centre of the crystal.
The second group of lamprophyric dike rocks (the camptonite monchiquite-alnoite series) is much less common than those above described. As a rule they occur together, and there are transitions between the different sub-groups as in the granito dioritic lamprophyres. In Sweden, Brazil, Portugal, Norway, the north of Scotland, Bohemia, Arkansas and other places this assemblage of rock types has been met with, always presenting nearly identical features. In most cases, though not in all, they have a close association with nepheline or leucite-syenites and similar rocks rich in alkalis. This indicates a genetic affinity like that which exists between the granites and the minettes, etc., and further proof of this connection is furnished by the occasional occurrence in those lamprophyres of leucite, hatiyne and other felspathoid minerals.
The camptonites (called after Campton, New Hampshire) are dark brown, nearly black, rocks, often with large hornblende phenocrysts. Their essential minerals are alkali hornblende and augite, olivine and plagioclase felspar. They have the porphyritic and panidiomorphic structures described in the rocks of the previous group, and like them also have an ocellar character, of ten very conspicuous under the microscope. The accessory minerals are biotite, apatite, iron oxides and analcite. They decompose readily and are then filled with carbonates. Many of these rocks prove on analysis to be exceedingly rich in titanium; they may contain 4 or 5% of titanium dioxide.
The monchiquites (called after the Serra de Monchique, Portu gal) are fine-grained and devoid of felspar. Their essential consti tuents are olivine and augite. Hornblende, like that of the camp tonites, occurs in many of them. Analcite is present in the base, either colourless or turbid through alteration. Some monchiquites contain hailyne ; while in others small leucites are found. Ocellar structure is occasionally present, though less marked than in the camptonites.
The alnoites (called after the island of Alno, Sweden) are rare rocks found in Sweden, Montreal and other parts of North Amer ica and in the north of Scotland. They contain olivine, augite, biotite, perofskite and melilite. They are free from felspar, and contain very low percentages of silica. The rocks from the type area and Montreal contain monticellite.
The chemical composition of some of these rocks will be indi cated by the analyses of certain well-known examples: In addition to the oxides given these rocks contain small quanti ties of water (combined and hygroscopic), S, MnO, etc.
(J. S. F.)