In the Rose-Tschermak-Brezina classification of meteorites the intermediate class of stony-irons, in which iron and stony matter occur in approximately equal amounts and to which Maskelyne gave the name siderolites, is divided into (1) siderolites proper, including the groups of the pallasites and the siderophyres, and (2) lithosiderites, comprising the groups of tile mesosiderites, glahamites, and ]odranites. Mesosiderites are defined as consisting of iron and crystalline olivine and bronzite, and grahamites of the same constituents with the addition of plagioclasefelspar.

The Amana meteorite, known also as Iowa County, Homestead, and West Liberty, is fully described in O. C. Farrington's 'Catalogue of the Meteorites of North America'. After the appearance of a brilliant meteor which on the night of February 12, 1875, passed with loud detonations from S.W. to N.E. over northern Missouri and southern Iowa, about 100 meteoric stones, weighing altogether some 500 lb., were found scattered over an elliptic area of about eighteen square miles from Amana in Amahs township to Boltonville in Iowa township, both in Iowa Co., Iowa.

The widespread basaltic rocks of tile Brito-krctic province, like those of the Deccan and Columbia basalt plateaux, do not seem to have received from petrologists the attention they deserve. Descriptions of particular localities and their rocks are not lacking, and the British region in particular has become classic through the investigations of Judd, Sir A. Geikie, and Dr. Harker, but hitherto no attempt has been made to survey the province as a whole, except in the most general way.

In the following notes I have endeavoured to give in a convenient and simple form a complete account of the relations connecting different twinning-laws or operations, which, when applied to the same crystal-structure, produce indistinguishable results.

A rigorous analysis of the possible relations between the compound structures of twin-crystals, based on the principles of the equality of the structural distances in the plane of composition, will be found in the author's papers, ' The geometry of twin-crystals' (Proc. Roy. Soc. Edinburgh, 1912-1918, vol. xxxii, pp. 416-432, 433-457) and 'Die Geometrie der Zwillingskrystalle' (Zeits. Kryst. Min., 1918, vol. lii, pp. 827-371). See also the paper 'Twin-planes and cross-planes' in this Magazine (1910, vol. xv, pp. 390-397).

Consider the poles of the faces of a crystal and the zones (great circles) which join them. Suppose we have given their gnomonic projection on to a certain tangent-plane to the fundamental sphere on which these poles and zones lie. We can find the gnomonic projection on any other plane by aid of the gnomonic net, as explained in this Magazine (1904, vol. xiv, p. 19); and similarly for the stereographic or orthographic projection.

Suppose a crystal has two good cleavages, and that a thin section is cut in a random direction. Cleavage-cracks are seen in the section which consist of two sets of parallel lines inclined at an acute angle ϕ, which may have any value whatever, depending on the direction in which the section is cut. The question suggests itself: what is the chance that ϕ may lie between assigned limits? This question proves to be incapable of a direct mathematical solution, but it can be solved graphically. This is worked out here for two typical cases, and the general method of performing the graphical solution in every case is indicated.

The pale celestite-blue calcite which occurs on the eastern half of Lot 13, Range V, of North Burgess township, Lanark county, Ontario, contains very remarkable lattice-like inclusions of a hydrous magnesium silicate.

The calcite forms crystalline masses belonging to the Grenville series, which has a very extensive areal development in certain parts of Ontario. The texture varies from medium to very coarse, in which the individual cleavage-rhombs of calcite measure one inch or more in diameter. Repeated twinning on e (0112) or (101) is almost invariably exhibited, and is especially pronounced in the more coarsely crystallized varietiea Both the twinning and the crystalline character are results of the very intense metamorphism to which this Archaean limestone has been subjected.