TEANSACTIONS OF SECTION C. 683 



some beiBOf readily decomposed, and others very permanent ; the indifference or 

 chemical resistance, moreover, increasincj with the hardness or mechanical resistance. 

 These two qualities vary, for species of analogous constitution, directly as their con- 

 densation ; while for species of similar condensation and hardness, the chemical in- 

 difference increases as alumina takes the place of the ordinary protoxyd-bases, lime, 

 magnesia, ferrous oxyd, and alkalies — a fact readily explained by the comparative 

 insolubility of alumina and aluminous silicates in atmospheric waters. The less 

 partial action of dilute fluorhydric acid on the various silicates shows more clearly 

 than the atmospheric process, the relation of condensation to chemical inditierence. 

 This relation may be made evident by a few examples. The condensation being 

 inversely as the so-called atomic volume, we find that when calculated by a simple 

 formula (elsewhere given by the author) for all silicates and oxyds this value, 

 represented by v {=p-^(l) for the various feldspars and scapolites, for nephelite, 

 iolite, and petalite, equals 6-8 -6-2; for the muscovitic or non-magnesian micas, 

 5-9 _ 5-6; for garnet, epidote, zoisite, and the various tourmalines, 5'4 — 5'.3 ; for 

 etaurolite and spodumene, 4'9 ; and for andalusite, topaz, fibrolite, and cyanite, 

 5'0 — 4'5, approximately. Comparing with these the common protoxyd-silicates, 

 we find for wollastonite and willemite, v = 6'6 ; for amphibole, 6"9 ; for pyroxene 

 and enstatite, 5-5 ; for chrysolite, 6--4 — 5-3; and for phenakite, 4-6. In the sub- 

 aerial decay of crystalline rocks, while felspars and scapolites among alumini- 

 ferous silicates are kaolinised, the micas, notwithstanding their laminated structure, 

 are much less readily changed, and garnet, epidote, tourmaline, andalusite, and 

 topaz are found unaltered, with the quartz, corundum, spinel, cassiterite, and mag- 

 netite left behind by the decay of the felspathic rocks — a process in which even 

 amphibole, pyroxene, and chrysolite share. ' The greater stability of those [sili- 

 cates] which belong to the more condensed types is shown in their superior 

 resistance to decay, and is thus of geological significance.' 



AVhile the above are examples of the varying resistance to the atmospheric in- 

 fluences of carbon dioxyd and water combined, other changes less well known take 

 place in sUicates by the subterranean action of watery solutions, where a greater 

 insolubility determines the formation of certain softer hydrated magnesian and 

 aluminous species by epigenesis from harder and more condensed species. The pro- 

 duction of these epigenic products, as was said in 1885, is due to their ' chemical 

 stability under the circumstances,' and it was added, ' The constancy in composi- 

 tion and the wide distribution of pinite show that it is a compound readily formed 

 and of great stability. Such being its character, it might be expected to occiu- as 

 a frequent product of the aqueous changes of other and less stable silicates. It is 

 met with in veinstones in the shape of crystals of nephelite, iolite, scapolite, fel- 

 spars, and spodumene, from each of which it is supposed to have been formed by 

 epigenesis. Its frequent occiu-rence as an epigenic product is one of the many 

 examples to be met with in the mineral kingdom of the law of " the survival of 

 the fittest." It is, however, difficult to a?sign such an origin to beds of this 

 [described as dysyntribite and parophite], which are probably the residts of original 

 deposition or of diagenesis.' 



Mr. E. A. Iiidsdale, who during the present year (1888) has done good service 

 by publishing a suggestive essay called * Notes on Inorganic Evolution,' speaks of 

 the production and conservation of more stable species, as above described, as a 

 gradual ' selection of inert forms,' and further, as ' a survival of the most inert.' 

 But as inertness consists in stability, and in fitness to resist alike the chemical and 

 the mechanical agencies which destroy other species, it is evident that his phraseo- 

 logy is but another statement of the formula of ' the survival of the fittest.' 



The great principle of the change of the mineral matters which existed in 

 former conditions of our planet, into other forms more stable under the altered con- 

 ditions of later ages, is but an extension to the mineral kingdom of the laws already 

 recognised in astronomical and biological development. As was written in 1884, 

 ' That a great law presided over the development of the crystalline rocks was from 

 the first my conviction, but until the confusion which a belief in the miracles of 

 metamorphism, metasomatism, and vulcanism had introduced into geology had 

 been dispelled, the discovery of such a law was impossible.' To this we may add 



