io8 American Quarterly Microscopical Journal. 



in one and the same rock are either all glassy or all fluid. It 

 therefore seems likely to me that in the porphyroidal rocks, the 

 fluid-inclusions are nothing else but original glass-inclusions, 

 out of which the glass-substance has been entirely decom- 

 posed and removed by aqueous solutions (p. 196)." 



On the other hand, Zirkel* maintains that "these forms cer- 

 tainly do not occur, as Vogelsang considers, 'in great predomi- 

 nance on cleavage-planes.' It may now and then be the case 

 that planes of the inclusions present themselves on the upper 

 surface of the thin section as extraordinarily fine cracks; in 

 such cases, nevertheless, the latter are certainly only second- 

 ary; the mineral bearing the inclusions is liable to fracture 

 most easily in that direction in which its continuity is most in- 

 terrupted, that in which the arrangement of its inclusions oc- 

 curs;" and again he states: "In all cases we must conclude 

 that the microscopic fluid-particles in different minerals, as 

 well as those included in the constituents of rocks, were en- 

 closed originally with their formation in a mechanical way." 



In opposition to the theory of Vogelsang that these liquid- 

 inclusions are cavities which have been filled by secondary in- 

 jection, Zirkel further calls attention to the absence of com- 

 municating fissures, none being detected under the highest 

 powers; the presence of bubbles in every {}) cavity of a 

 group; the hermetical sealing up of the cavities, so that in 

 experiment a strong heat is required to produce the expulsion 

 of the liquid, with decrepitation; and the chemical nature of 

 the liquids. Therefore, according to the generally accepted 

 opinion on the subject, in all cases rocks have been formed in 

 presence of, or under saturation by, these substances in liquid 

 or gaseous form. 



The character and identity of the liquid and gases which 

 occupy these cavities are determined by certain chemical ex- 

 periments, founded substantially upon their expulsion by ig- 

 nition in a miniature glass retort, and their subsequent be- 

 havior, on introduction into a solution of some proper reagent 

 {e.g. Baryta-water, for the determination of carbonic acid) or 

 into a Geissler-tube for Spectroscopic indications. 



Another simple, but important . method is commonly 

 employed for the same purpose. It is found that on 

 warming a thin section of rock or mineral containing 



*Mikr. Beschaflf d. Min. u. Gesteine, p. 47, note. 



