446 



NA TURE 



{March 12, 1S85 



give the name [diabase. We are inclined, on the other hand, 

 to call the rock a dolerite. The important point for the student 

 to remember, however, is that in the present unsettled state of 

 nomenclature his primary duty is to make himself familiar with 

 the structure and composition of the various rock types. The 

 question of names is, after all, only of secondary importance, 

 provided we remember that in looking at the facts through the 

 medium of an unphilosophical nomenclature we may so distort 

 them as to fail to realise their true forms and relations. 



Consider now the ^etiological aspect of petrography. Most of 

 us are so constituted that we cannot rest satisfied with a mere 

 description of facts ; we almost instinctively endeavour to dis- 

 cover what we call the origin of things. This, after all, merely 

 consists in tracing 1 ack as far as possible the chain of events of 

 which the object or phenomenon in question represents the last 

 link. The search after causal relations in the organic world has 

 led to the introduction of a principle which is now recognised as 

 one-of the greatest importance in almost every branch of human 

 knowledge. Changes in the characters of organisms are now 

 admitted to be determined by two factors — the inherent 

 properties of the organism and the influence of surrounding cir- 

 cumstances. A very little consideration will serve to show that 

 the changes which occur during and subsequent to the deve- 

 lopment of minerals and rocks are determined by two allied 

 factors. 



Take the case of crystallogenesis. It is not difficult to see 

 in a general kind of way that the characters which a crystal pos- 

 sesses have been determined (i) by the inherent properties of the 

 crystallising substance, and (2) by the influence of surrounding 

 circumstances — of the environment. When we examine thin 

 sections of rocks, furnace-slags, or the refuse products of glass- 

 works, we frequently find a number of bodies which are inter- 

 mediate as it were between glass and true crystals. These have 

 been carefully examined and admirably described by Hermann 

 Vogelsang, who has also succeeded in producing many of them 

 by artificial means. As they serve to illustrate in a very striking 

 way the principle above referred to, a short description of 

 Vogelsang's experiments will not be out of place. 



The crystallising substance finally selected by Vogelsang for 

 the purpose of his experiments was sulphur. This substance is 

 readily soluble in bisulphide of carbon, out of which it crystal- 

 lises in the rhombic form. If the process of crystallisation be 

 followed under the microscope, nothing definite as to the nature 

 of crystalline growth can be made out." The first objects which 

 appear are definite crystals, and these grow by accretion. If, 

 however, the solution of sulphur be thickened with Canada 

 balsam then, provided the proper proportions of the different 

 substances have been employed, some very interesting pheno- 

 mena may be observed by the aid of the microscope as the 

 bisulphide of carbon evaporates. Minute fluid spheres arise in 

 the medium and grow by mutual absorption. They finally con- 

 solidate as clear, transparent, isotropic bodies, and to them 

 Vogelsang has applied the term globulites. It is impossible to 

 determine absolutely the composition of these globulites, but 

 there seems no reason to doubt the conclusion of Vogelsang that 

 they are portions of the Canada balsam which are richer in 

 sulphur than the surrounding mass, and that they arise in conse- 

 quence of the attempt of sulphur to crystallise under unfavour- 

 able circumstance.-. Similar bodies may be observed in certain 

 rocks, slags, and blow-pipe beads, although the crystallising 

 compounds must be very different in the different cases. 



Under certain circumstances the mass of sulphur and Canada 

 balsam solidifies with the formation of globulites, but under 

 other circumstances additional phenomena may be observed. 

 When the resistance of the medium is too great to prevent the 

 union of the globulites, but not too great to prevent their ap- 

 proach, they become united into various more or less definite 

 forms. The mode of union depends partly on the way in v. 

 the globulites attract each other, and partly on the movements 

 in the mass. A linear arrangement of the globulites is very 

 common, and to the form arising in this way Vogelsang has given 

 the name margarite. A rectangular grouping is also not un- 

 common. From a study of the various forms arising in conse- 

 quence of the union of globulites, Vogelsang concludes that in 

 the case of sulphur there are in each globulite, as it were, three 

 directions at right angles to each other, in which the attraction 

 is considerable, and that in one of these the attraction is 

 decidedly greater than in the other two. 



The building up of the compound forms naturally leaves the 

 surrounding space free from globulites. 



Under certain circumstances the globulites become fused, as 

 it were, at the points of contact. This occurs when the resist- 

 ance is sufficient to prevent the assumption of the spin 

 but not sufficient to resist the destruction of the pellicle at the 

 point of contact. In this way rod-like bodies, termed longulites, 

 arise. 



It must be remembered that all these forms are strictly iso- 

 tropic. They are not, therefore, in any sense of the word, 

 crystals. The moment a true crystal of sulphur appears, it can 

 be recognised by its doubly-refracting properties. They have 

 been termed crystallites, wherever they occur, by \ r 

 and they evidently arise in consequence of the attempts of some 

 definite chemical compound to crystallise under conditions which 

 do not admit of the free approach of the molecules. 



Between crystallites and perfect crystals showing definite ex- 

 ternal faces there are numerous intermediate forms, such as 

 microlites and skeleton crystals. As further illustrations of the 

 influence of the environment we have only to consider the facts 

 that no two crystals of the same substance are precisely alike in 

 all their characters, and that some substances, like sulphur and 

 carbonate of lime, may be made to crystallise in tsvo different 

 systems by varying the conditions under which the crystallisation 

 is effected. 



There can be no doubt, then, that two factor- are involved in 

 the determination of the properties which crystals present : the 

 inherent forces of the crystallising substance and the influence 

 of surrounding substances. 



So far we have referred only to the birth and growth of crys- 

 tals. But the history of a crystal does not cease with it- forma- 

 tion. With a change in the surrounding circumstances the 

 crystal may be modified or destroyed. Thus we see that crys- 

 tals have a kind of life-history : they are born, they grow in 

 size by accretion, and finally they cease to exist as distinct 

 individuals. 



As an illustration of the influence of a change of physical 

 condition on the character of a crystal, consider the case of 

 leucite. At ordinary temperatures this mineral is generally re- 

 garded a- tetragonal, and it certainly shows double refraction in 

 thin sections. Klein has shown that by heating leucite to a 

 point far short of its fusibility it may be rendered perfectly 

 isotropic, and hence follows the conclusion that leucite is leally 

 isotropic when subject to the conditions underwhich it i- formed. 

 That crystals should be in a state of stable equilibrium, so far 

 as molecular forces are concerned, when subject to the J 



conditions under which they are formed, is exactly what we 

 should expect, and that this equilibrium maybe disturbed by 

 a change in these conditions is also very easy to understand. 



As further illustrations of the principle here referred to, consider 

 the various cases of paramorphosis, such as the change from 

 arragonite to calcite, or from calcite to arragonite ; or, again, 

 the con ponding changes in sulphur. 



Illustrations of the changes which arise in crystals in conse- 

 quence of changes in the chemical conditions to which they are 

 subjected, need not here be referred to in detail : the destruction 

 of crystalline rocks by denudation is of course a consequence of 

 these changes. 



Consider, now, the rocks of which the earth's crust is com- 

 posed. They also have a life-history. They are formed and 

 destroyed, and it is the business of the petrographer not only to 

 describe and classify them, but also to trace out the cycle of 

 change. For the purpose of illustrating this branch of petro- 

 graphy let us consider certain facts with reference to the genesis 

 of crystalline igneous rocks. It will lie admitted on all hands 

 that such rocks as granite, syenite, diabase, rhyolite, trachyte, 

 andesite, and basalt have originated by the consolidation of an 

 intensely heated silicate-magma under different conditions as to 

 temperature and pressure. The consolidation has been accom- 

 panied — except in those cases where the magma has con- 

 solidated as a homogeneous glass, and these will be left out of 

 account for the present — by the development of crystals. If, 

 then, we would understand the manner in which crystalline 

 igneous rocks have been formed, we must consider the subject 

 of crystallogenesis in silicate-magmas. Numberless facts which 

 need not here be referred to prove that the process of consolida- 

 tion is not a sudden 1 ne. A- the surrounding circumstances 

 (environment) become more and more favourable to crystallisa- 

 tion, the minerals separate out one after the other, and at last 

 the whole mass becomes solid, and the rock is forme 1 he 



temperature at which any given mineral forms is not determined 

 by its own fusibility. The law, of the formation of minerals in 



