page 63 note * These are the crystalline forms of tetradymite.

page 65 note * M. Delesse'e memoir has been often referred to, and was particularly ably summed up by the President of the Geological Society in his last anniversary address, which should be consulted by all who wish to read a comprehensive review of the recent inquiries on this subject.

page 66 note * Trachyte is a rock containing orthose and anorthose felspar, ferro-magnesian mica, hornblende, and also quarts. By anorthose felspar is designated, in a general way, all the felspar species belonging to the sixth crystalline system. Dolerite is an anorthose and anhydrous lava, composed of anorthose felspar and augite, with sometimes olivine, mica, and lencite.

page 66 note † Retinite consists of vitreous orthose felspar, of ferro-magnesian mica, and also quartz rather rarely, and always a large proportion of water, which may rise from 10 to 100 per cent. Pechstein and phonolite are varieties of this rock. Basalt principally consists of anorthose felspar, augite, and olivine, with sometimes protoxide of iron, carbonates, zeolites; and accidentally nephetine, haüyne, zircon, corundum, &c., forming a hydrated felspathic paste. Basalt has the same alementary composition as dolerite, differing principally in the presence of a certain quantity of water and volatile matters.

Trap, properly so-called, may pass into basalt, with which it is often associated. An intimate relation exists between the two rocks, but heat only played a very minor part in the formation of trap. Its base is anorthose felspar, which is generally the only mineral possible to be recognized; this is always hydrated, often considerably. It is rich in oxide of iron, and often contains spathic carbonates. Trap forms the limit of the pseudo-igneous rooks; and although it is intimately connected with basalt, it differs I think in having been formed in a lower temperature.

page 67 note * Granite, as a type of a large class of rooks, has almost the same mineral composition as trachyte, for it contains quarts, felspars, and micas; but the occurrence and characteristics of these minerals are very different. Its quarts is particularly worthy of remark. In rooks of igneons origin this mineral is often entirely wanting; now, in granite, on the contrary, it is very abundant, amounting to even a moiety in certein rooks, yet the total quantity of silicic acid present is not greater than in trachyte; the greater abundance of free quarts being due to the facilities which the mode of origin of granite afforded this mineral of separating itself from the magma. It must be borne in mind also that this quarts is not only always crystalline, but also always hyaline. The study of the felspars of granite is also very instructive. They are orthose and anorthose, opaque, or at most translucent, never vitreous, and always contain a certain quantity of water, usually trifling in the orthose, but varying from 2 to 100 percent. in the anorthose. The consideration of the micas and other minerals of this rock are equally instructive in showing its aqueous origin.

Diorite has a very simple mineralogical composition, being essentially formed of anorthose and hornblende, with, at times, protoxide of iron, sphene, ferro-magnesian mica, and, accidentally, garnet. Diorite greatly approaches granite in its mineral composition. Its metamorphism is analogous, and the one may pass insensibly into the other. Diorite may be considered as formed under conditions intermediate between those that have produced trap and granite.

Serpentine has hitherto, of all the eruptive rocks, been the most enigmatical. Its mineralogical characters are so well known, that it is unnecessary to repeat them; but it is particularly distinguished by its large per centage of water, from 18 to 100 per cent In this rock all effects of heat have entirely disappeared, and its plasticity can scarcely be attributable to any other causes than water and pressure.