The products of the metasomatism of igneous rocks are found for the most part only in small quantities since diffusion through a solid rock of the solutions responsible for metasomatic changes is a slow process. In the Whin Sill there are two types of metasomatic alteration product which though insignificant in total amount are nevertheless widely distributed. The first type, which was formed along the walls of an early joint system by the action of juvenile solutions, is part of the history of the cooling down of the Whin Sill and will be described in a subsequent paper. The second type was produced by the action of lead vein solutions. Although unconnected with any stage in the cooling down of the Whin Sill, this type has an important bearing on the general mechanism of replacement and especially on the part played by diffusion, whose effects may be seen in both types as an extended transition between altered and unaltered dolerite.

It is now eight years since the present writer drew attention in these pages to his discovery of a diamond-bearing pipe of igneous origin in Brazil (3). The matter, though hotly contested and denied at the time, seems now to have been generally accepted. So much so in fact that Dr. Alex. du Toit, in his presidential address to the Geological Society of South Africa (10) compares the diamond-bearing pipes of Brazil and South Africa as an indication of the former close connection of these lands.

In most systems of petrography the glassy rocks are treated in a very casual way. The names which are commonly given to them, such as obsidian, pitchstone, perlite, vitrophyre, and the like, afford no reliable indication of composition; and when a rock is partly crystalline and partly glassy, the composition of the glass is often assumed, quite unwarrantably, to be the same as that of the crystals. It is only in the Norm classification that vitreous rocks fall unfailingly into the same compartments as holocrystalline ones of similar chemical composition; and before any rock can be classified by this method, it is necessary to have a complete chemical analysis of it; which is, for many workers, a serious obstacle.

The Carnic Alps, the summit-ridge of which is the frontier between Austria and Italy, are famous for their Palaeozoic formations, which contain large and beautiful faunas of Ordovician, Silurian, Devonian, Upper Carboniferous, and Permian age. The Carnic Alps form a very important part of the whole Alpine region, because they display clear evidence of Palaeozoic orogenesis; for example, the Taconic orogenesis between Ordovician and Silurian (which is of relatively little importance), and some phases of the Variscan orogenesis (between Devonian and Upper Carboniferous, and between the limestone of Trogkofel, which is equivalent to the stage of Artinsk, and the Lower Permian).