IDG CHEMISTRY OF THE EARTH. 



lixiviated rocks tbe proportion of alkali will be nearly or quite sufficient 

 to form ortlioclase or albite with the whole of the alumina present, bat 

 as the alkali diminishes, a portion of the alumina will crystallize, upon 

 the metamorphism of the sediments, in the form of a potash-mica, such as 

 muscovite or margarodite. While the oxygen-ratio between the alum- 

 ina and the alkaliin the feldspars just named is 3 : 1, it becomes C : 1 in 

 margarodite and 12 : 1 in muscovite. The appearance of these micas in an 

 aluminous rock denotes, then, a diminution in the amount of alkali, until 

 in some strata the feldspar almost entirely disappears, and the rock be- 

 comes a quartzose mica-schist. In sediments still further deprived of 

 alkali, metamorphism gives rise to schists tilled with crystals of kyanite or 

 of andalusite, simple silicates of alumina, into which alkalies do not en- 

 ter, at least in noticeable quantities; but, in case the sediment still retains 

 oxide of iron, staurolite and iron-garnet take their place. The matrix 

 of all these minerals is generally a micaceous schist. The last term in 

 this exhaustive process appears to be represented by the disthene and 

 pj^'ophyllite rocks which occur in some regions of crystalline schists. 

 In conformity with what has just been pointed out, it will be seen that 

 these aluminous silicates destitute of alkalies do not occur in the oldest 

 known sediments; in those of the Laurentian system, in which also mica 

 is found in comparatively small quantities, nearly all the alumina present 

 being in the form of ortlioclase or albite.* 



§ 30. By metamoridiism in geology is understood the change of chem- 

 ical and mechanical sedimentary deposits into crystalline stratified 

 rocks. The conversion of these sediments into definite mineral species 

 has been effected in two ways: First, by molecular changes — that is to 

 say, by a crystalline arrangement of particles of definite compounds 

 previously formed ; and, secondly, by chemical reactions between the ele- 

 ments in heterogeneous sediments, giving rise to new compounds, which 

 become crystalline in their turn. Pseudomorphism, which is the change 

 of one mineral species into another hj the introduction or the elimina- 

 tion of some element or elements, presupposes metamorphism, since only 

 definite mineral species can be the subjects of this process. To confound 

 metamorphism with pseudomorphism, as Bischof and others after him 

 have done, is therefore an error. It may be further remarked that 

 although certain pseudomorphic changes may take place in some min- 

 eral species existing in veins and near to exposed surfaces, the alteration 

 of great masses of silicatcd rocks by such a process is as yet an un])roved 

 hypothesis. 



§ 31. The cases of local metamorphism in proximity to intrusive rocks 

 go far to show, in opposition to the views of certain geologists, that 

 heat has been one of the necessary conditions of the chemical change. 

 The source of this heat is generally admitted to be from below, but to 

 the hypothesis of alteration by ascending heat Naumann has objected 

 that the inferior strata in some cases escape change, and that, in descend- 

 ing, a certain plane limits the metamorphism, separating the altered 

 strata above from the unaltered strata beneath, there being no apparent 

 transition between the two. Tliis, taken in connection with the well- 

 known fact that in many cases the intrusion of igneous rocks causes no 

 apparent change in the adjacent unaltered sediments, shows that heat 

 and moisture are not the only conditions of metamorphism. I showed, 

 by experiments in 1857, that, in addition to these conditions, certain 



* For a discnssion of tills subject seo my paper on The Chemical and Miucralogical 

 Relations of Metamorphic Roclcs, Dublin Quarterly Journal of Science, July, ISiJo; also 

 Geology of Canada, 1863, page 561, and chap. XIX, of the same work. 



