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resembles the cooling and evaporation of a saline solution, the gases playing the part of solvent. The minerals appear in the order of their insolubility. It is probable that the history of magmas will never be clearly understood^ till a very careful study is made of the consolida- tion of rock-making silicates under high pressures of steam and other gases such as are known to abound in natural volcanic magmas.

Classification of Magmas. The igneous rocks of one geological period and province have often so many peculiarities in common that they can be regarded as having resulted from the consolida- tion of a single reservoir of molten matter. The chain of volcanoes that fringes the shores of the Pacific Ocean from Tierra del Fuego to Alaska, and thence by Japan and the Philippines to Java and Sumatra, is characterized by rocks which have so much similarity in many important characters that they are certainly of allied origin, even if they have not proceeded from the one source. These rocks are all of Tertiary and recent age; their eruptions began in Eocene or Miocene time and have continued, with more or less frequent intermissions, up to the present day. For another example of this we may take the igneous rocks of the western and mid-Atlantic area, from Jan Mayen, through Iceland, the Heb- rides, Canaries, Cape Verde Is., etc. All these volcanic centres have many rock types in common, and the whole assemblage is strikingly different from the Pacific igneous rocks. Each of these magmas has been taken as a type, and it has been found that in the older geological periods they are also represented; for exam- ple, the early Devonian eruptions in Scotland are distinctly of the Pacific type, while the Carboniferous eruptions in the same district are of the Atlantic type. If we seek for a precise defini- tion of their respective characters it is not easy to give a com- plete answer. It may be said, however, that the Pacific suite has a great prevalence of hypersthene andesites, and andesites of all kinds. The Atlantic lavas, on the other hand, are predominantly olivine basalts, with trachytes and phonolites. Another feature which is especially striking is that practically all the rocks carry- ing nepheline and other felspathoids or "alkali minerals "are found in the Atlantic suites. This has been regarded as proving that the Atlantic magmas are richer in alkalis and the Pacific in lime, but it is by no means certain that this is the explanation. In fact, a full chemical discussion of the relations of these rock- series to one another has yet to be undertaken, but from the work of Becke it seems that the Pacific are essentially richer in silica, and in the " light " elements generally, while the Atlantic con- tain more of the " heavy " elements, such as magnesia, iron, chromium, titanium. Several authors have pointed out that the rocks of the Pacific group are associated with a folded mountain chain and consequently have appeared in a region undergoing lateral compression and upheaval ; the Atlantic, on the other hand, are associated with a region of subsidence, with vertical disloca- tions along lines of fissure and faulting in other words, a region subjected to lateral tension and depression. A third group of igneous rocks, very well characterized and distinct in many respects, is the pillow lavas or spilites, which are perhaps the most abundant volcanic rocks of the earlier geological periods and are very widespread in the Lake Superior district, middle Europe, Wales and Scotland. Among these lavas types rich in soda are common and albitization is a frequent pneumatolytic change. These rocks seem to accompany depressions formed in conse- quence of folding.

The attempt to classify volcanic magmas in groups which accompany definite types of earth movements, folding and fault- ing, is exceedingly fascinating. A general survey of the world's volcanics from this standpoint has been undertaken by Iddings and Stark, but the results are by no means conclusive. A great deal has yet to be done before we understand the full range of variation of many local magmas, their relation to one another and the approximate age of the intrusive types. It seems, however, that Atlantic, Pacific and spilitic suites are not in all cases sharply distinct. Harker believes, for example, that the Hebridean Tertiary magmas of Scotland are Atlantic with some Pacific affinities. He had formerly classified them as Pacific. Bailey and Clough have found in the same district a group of pillow lavas occupying a central volcanic subsidence or " caldera." In that case, accordingly, all three types occur in one narrowly cir-

cumscribed area. If so, they cannot be regarded as distinct types of magma, but rather as facies which may be developed or may appear as offshoots of a magma. Their connexion with definite types of telluric movement becomes doubtful. American petrol- ogists also, from a study of the Tertiary magmas of the Western States, are by no means satisfied that the alkali rocks, such as phonolites, tephrites and leucitic lavas, may not belong to a definitely Pacific assemblage. In Australia, Atlantic and Pacific magmas seem to be intermixed, but in Africa Atlantic igneous rocks accompany fault-fissuring, and this extends into Arabia and Palestine. There is no doubt that on a broad scale the Atlan- tic and Pacific types have appeared again and again in the earth's history, and have maintained their distinctive characters. The distinction, however, is not absolute, and the rule has many exceptions. Recognition of a number of additional types has recently been proposed, and along this line it is probable that there may be considerable advance in the near future.

Experiments on Constitution of Binary Magmas. A molten rock magma may be regarded as a liquid composed principally of oxides (mostly silicates). It crystallizes from a variety of causes, of which cooling is the most important, though relief of pressure and escape of gases may also play a part.

The laws followed in such a case have been very carefully in- vestigated, not only for metals, salts and organic compounds but also for many minerals. It is generally true that a mixture of two substances will have a lower consolidation point than one of the pure substances, and, if mixed in certain proportions, they will consolidate at lower temperatures than either of the components. Thus salt and ice, if mixed in the solid state at temperatures a little below the freezing-point of water, will melt, forming a liquid which is colder than the ice originally taken; and aqueous solutions of salts have always a freezing-point lower than that of pure water. For each pair of substances there is a definite mixture which has the lowest tem- perature of consolidation and this is known as the eutectic mixture.

In the diagram (fig. i) the horizontal coordinate repre- sents composition, the ver- tical representstemperature. A mixture of any given com- position is represented by a vertical line and different points in that line represent different temperatures of the mixture. The two slop- ing lines AE and EB divide the diagra m into two regions, of which the uppermost in- dicates substances in a pure- ly liquid state; below the line the substance is in part or wholly in a solid condition. A vertical (composition) line, accordingly, when it

cuts these lines shows where a substance begins to crystallize. The point where the curves meet is the eutectic point E, and shows the composition of the mixture which has the lowest freezing tempera- ture and the temperature at which it consolidates. The horizontal line drawn through the eutectic point separates the diagram into two regions, a lower one in which the substance is entirely solid and an upper one in which liquid is present. If we take any vertical line in the diagram, it will indicate a mixture of definite composi- tion ST and, followed downwards, it shows the changes taking place in such a mixture as the temperature falls. Above the line AE the mixture is a cooling liquid. At S crystallization begins. At T the last liquid portion disappears and consolidation is complete ; below T the substance is a cooling solid. The diagram refers only to substances that crystallize on solidifying; glasses are solids which essentially resemble highly viscous liquids in their properties. When crystallization begins the substance which is in excess of the eu- tectic mixture will crystallize out first, the residual liquid becom- ing poorer in that component until it has reached the eutectic composition, when the two components will go on crystallizine simultaneously till all is solid. The composition of the liquid will travel along the line AE from S to the point E, where it will remain constant.

Such a diagram is based on a series of experiments in which a known mixture of two substances (very carefully purified) is heated in a furnace (generally electric) to a temperature well above its melting- point. The mixture is then allowed to cool slowly and steadily, and its temperature recorded at short intervals or continuously by some form of pyrometer or recording thermometer. The rate of cooling can be very accurately ascertained and plotted (fig. 2). Crystalliza- tion is attended by liberation of heat (the liquid losing its latent heat as it passes into a solid), and this involves a retardation of cooling. The simple liquid cools at a uniform rate; crystallization

COMPOSITION Fl G. I

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