122 G. F. Becker — A new law of Thermo- Chemistry. 



quence, for it can hardly be doubted that a simple substitution 

 in an existing compound will, as a rule, take place more rap- 

 idly than a fundamental re-arrangement of atoms. 



I shall have occasion hereafter to call attention to instances 

 in which, as nearly as can be ascertained without exact deter- 

 minations of rate, the order of ordinary laboratory reactions 

 conforms to the law here announced. I desire, however, to 

 point out without delay its applicability to a problem of geo- 

 logical chemistry, in which exact and direct experimentation 

 is almost or quite impossible, viz : the order of the genetic suc- 

 cession of minerals in massive rocks. This is a subject of the 

 greatest importance to rock classification and to the whole sci- 

 ence of lithology. It is also one in which little or no advance 

 has been made. 



No a priori guess could be more natural than that the order 

 of succession of minerals in eruptive rocks is that of their fusi- 

 bility. Indeed, this corresponds closely to the old empirical 

 (and incorrect) rule of chemistry, according to which precipita- 

 tion takes place whenever an insoluble compound would result 

 from any re-arrangement of molecules. Nothing is more easily 

 confuted, however, than this hypothesis of genetic succession, 

 if rigidly interpreted, since magnetite, for instance, is among 

 the first and among the last minerals to form in several erup- 

 tive rocks. Bunsen, too, has shown that the temperature at 

 which an isolated body solidifies is never that at which it sepa- 

 rates in the. solid state from its solutions.* This fact does not 

 appear to me to show that the order of succession has no con- 

 nection with the fusibility, as is sometimes stated, f but only 

 that if such a connection exists, it-is not an entirely simple one. 

 There are coincidences between the order of succession and the 

 order of fusibility of the mineral species composing massive 

 rocks which it is scarcely possible to regard as accidental. Thus 

 zircon and olivine are among the first minerals to solidify, and 

 their only common characteristic appears to be a high degree of 

 infusibility. The highly refractory quartz, too, is an early 

 mineral in the quartzose porphyries, and sometimes, also, in 

 granite. Messrs. Fouque and Michel-Levy were, I believe, the 

 first to show \ that in the following triclinic feldspar series ; 

 anorthite, labradorite, oligoclase ; the crystals of primary con- 

 solidation almost always precede the ciwstals of secondary con- 

 solidation. They have also shown§ that this is the order of 

 the fusibility of these minerals, while Professor Szabo finds 

 that bytownite and andesine occupy a position in the scale of 

 fusibility similar to that which they occupy in the scheme of 

 chemical composition. 



*Zschr. G-eol. Ges., 1S61. + Roth, Allgemeine u. chem. Geol., vol. ii, p. 48. 

 % Min. microgr. gComptes Rend., T.8T8. 



