Introduction.—The conditions described below were observed over a large part of South-West Texas; but the local details apply to an area of one or two hundred square miles in McMullen County, geologically surveyed by the writer in 1912. The centre of observations was a little mining ranch named Crowther, about 50 miles south of San Antonio and 80 miles from the Gulf of Mexico, near the inland limit of the Gulf coastal plain. The country for many miles around Crowther is practically untouched by man, though a somewhat primitive little local railroad now under reconstruction reaches to within 16 miles of it.

At the Stockholm meeting of the Congress in 1910 an invitation to hold the twelfth meeting in Canada was accepted. As the Congress met in the United States in 1891 and in Mexico in 1906, members were thus afforded an opportunity of visiting all the great divisions of North America. The Canadian meeting was held at Toronto from August 7 to 14, 1913, under the presidency of Professor F. D. Adams, of McGill University. About 600 members attended it, although the total enrolled was nearly twice as great, and 46 countries were represented among the members. The Congress was formally opened by the Right Hon. Sir Charles Fitzpatrick, on behalf of H.R.H. the Duke of Connaught, the Honorary President, who was unavoidably absent, and speeches of welcome were made by others. Dr. R. W. Brock, Director of the Geological Survey of Canada and General Secretary of the Congress, presented to the Congress a monograph entitled “The Coal Resources of the World”, the result of an inquiry made upon the initiative of the Executive Committee of the Twelfth Congress, with the assistance of Geological Surveys and mining geologists of different countries. It consists of three quarto volumes of about 400 pages each (11 by 8 1¼4 inches) and an atlas of 66 pages of maps in colours (13 1¼2 by 191¼2 inches) published by Morang & Co., of Toronto, at $25 per set, net.

It is now time to turn to the consideration of the part played by these organisms in the formation of sedimentary rocks through the successive geological periods.

This complex of igneous rocks lies between Ardlui, at the head of Loch Lomond, and the head of Loch Fyne. In 1892 it was the subject of a fairly exhaustive paper by Teall & Dakyns, but in view of a number of new facts which we have discovered we venture to submit a re-examination of the problems connected with the complex. The mass is broken by a north-east fault. The country to the west of this fault is mainly porphyritic granite and tonalite; to the east there is tonalite and also more basic diorite and ultrabasic rocks. We shall concern ourselves mainly with the basic and ultrabasic groups, as they are somewhat unusual in the “Newer Plutonic Rocks of the Highlands”.

Of the various minerals which enter into the composition of igneous rocks, about one-half are capable of forming in presence of free silica, as is shown by their association in rocks with quartz and tridymite. These may, for the present purpose, be termed saturated minerals; they include some of the most highly silicated compounds of their respective metallic elements. The remaining rock minerals do not appear in association with free silica, and may be presumed to be incapable of stable existence in its presence; they are mostly the less highly silicated compounds of the same metallic elements which, enter into the former group. These may, for the present purpose, be, termed unsaturated minerals. The unsaturated character of any mineral in the latter group is not affected by the temperature and pressure at which crystallization takes place; this is shown by the fact that quartz and the minerals referred to are mutually exclusive both in plutonic and in effusive rocks. In the case of sodium, potassium, calcium, and magnesium the formation of saturated or unsaturated minerals appears to depend only on the amount of silica available in the magma. Aluminium, ferrosum, and ferricum, elements of feebly basic character, have their combinations largely determined for them by the more strongly basic elements, with which they tend to form either complex molecules or mix-crystals.