A great granite batholith, which may be many miles in diameter, is often surrounded by a wide aureole of contact alteration, varying from a few hundred yards to two miles in breadth. This variation may have structural causes dependent on the under ground contour of the intrusion.
Crystalline schists which are already recrystallized may show little sign of alteration, or are affected only at the immediate contact, and the same remark applies to fresh igneous rocks which have consolidated at high temperatures. If, however, they have been subject to alteration by weathering, or contain amygdales filled with hydrated minerals such as zeolites, chlorite, etc., funda mental changes may be produced. The altered rocks of such aureoles are known as hornfelses (q.v.), and are typically fine grained compact rocks devoid of fissility and cleavage. Argil laceous sediments give rise to dark lustrous hornfelses full of minute scales of red-brown biotite and cordierite, limestones to marbles; impure limestones become grey, yellow or green calc silicate hornfelses rich in diopside, grossular garnet, wollasto nite and vesuvianite; while dolerites, basalts and andesites are transformed into dark granular hornfelses with a large develop ment of new pyroxene, hornblende, biotite and recrystallized felspar. Cherts, flints and fine sandstones are converted into quartzites, consisting of small close-fitting grains of quartz.
The progressive changes that take place in an aureole may be determined by studying the effects observed at the outer edge of the aureole, and tracing these changes inwards to the contact. Argillaceous rocks are very suitable for this purpose on account of their common occurrence and homogeneity. In most aureoles the first signs of alterations in shales or slates is seen in an in duration or hardening of the rock, accompanied by the develop ment of minute spots, which consist either of new-formed minerals or a new distribution of minerals. Considerable alteration may be effected without deleting other structures. Fossils may be in part preserved and are not destroyed till the whole rock has been recrystallized. This variation in the nature of the incipient changes is probably to be ascribed to the mineralogical nature of the argillaceous rocks themselves. All shales and slates are
built up of varying amounts of quartz, sericite, chlorite, iron oxides, kaolin or other hydrated aluminium silicates. Where the last named minerals are abundant, an early formation of andalu site and cordierite is to be expected. Where they are scarce or absent, the first signs of change are seen in the rearrangement of the original minerals or the development of new-formed biotite.
Regarding a metamorphic rock as a mineral assemblage formed within a given temperature and pressure range, it is clear from physicochemical principles that the number of co-existing mineral phases in equilibrium must be limited by the number of com ponents. It is to the completely recrystallized products of the hornfels zone that the phase rule of Willard Gibbs may be applied. The measure of success obtained by its application to meta morphic mineral assemblages, and so the utility of an ideal classification of metamorphic rocks developed with its aid, are largely dependent on the approach towards equilibrium of the final products. Actual experience, indeed, has afforded a very considerable degree of confirmation of the results to be expected.
Briefly, the phase rule states that the number of phases plus the number of degrees of freedom exceeds the number of com ponents by 2, p+ f =c+ 2. For example: if one takes into consid eration a three-component system, five solid phases can at most co-exist. Under such conditions both temperature and pressure are fixed. Metamorphic processes, however, take place over a range of temperatures and pressures, and the random conditions attendant upon particular examples of metamorphism imply a divariant system, so that the maximum number of solid phases in a three component system will not exceed three. As expressed by Goldschmidt, the phase rule applied to mineralogical systems can be stated as "the maximum number of solid minerals that can co-exist in stable equilibrium is equal to the number of individual components that are contained in the minerals if the singular temperatures of transition points are excluded." This important rule has been a very useful guide in the study and classification of metamorphic rocks, and it will be of interest to note its application to some simplified rock systems.