19 



(7) Crystals containing liquid and glass (or stone) inclusions were 



formed under great pressure by the combined action of igneous 

 fusion and water. 



(8) Crystals having the characters of 6 and 7 combined were formed 



under great pressure by the united action of igneous fusion and 

 water, alternating with vapour and gas, so as to include all the 

 conditions of igneous fusion, aqueous solution and gaseous 

 sublimation. 



The above conclusions are unquestionably of great value and interest ; 

 at the same time it must be admitted that when we attempt to apply them to 

 the minerals of eruptive rocks, for the purpose of ascertaining how those rocks 

 were formed, we are brought face to face with very great difficulties. They 

 can only be applied, for instance, in cases where there is no doubt as to the 

 original character of the minerals and the inclusions. It is assumed, in the 

 above statement, that all inclusions are original. Subsequent research has, 

 however, tended to show that secondary inclusions are by no means uncommon. 

 The different kinds of natural inclusions will now be described, with special 

 reference to the igneous rocks. 



GLASS INCLUSIONS. Glass inclusions are especially abundant in the 

 porphyritic crystals of volcanic rocks ; as, for instance in the felspars of 

 rhyolites, andesites, and porphyritic basalts. In these cases they may, 

 without doubt, be regarded as original ; or, in other words, as owing their 

 origin to the enclosure of portions of the mother-liquor during the growth of 

 the crystal, and the subsequent consolidation of this as the rock cooled down 

 to the ordinary temperature. 



These inclusions are especially common in those rocks which contain a 

 glassy base. Sometimes the glass of the inclusions is similar to that forming 

 the base of the rock. This is the case with the inclusions in the sanidine 

 crystals of the Arran Pitchstone. (1) Sometimes, on the other hand, the glass of 



Fig. 8.(2) Glass-inclusions in the felspar of a porphyritic pitchstone from Arran, containing 



microlites and a bubble. Magnified 300 diameters. 

 Fig. 9. Ditto, containing several bubbles. Magnified 330 diameters 

 Fig. 10. Ditto, with a distorted bubble, into which the microlites project. Magnified 600 



diameters. 



the inclusions is darker in colour than that of the rock ; (8) a fact which may 

 be taken as indicating a difference in chemical composition, and probably a 



(1) SOEBY. Microscopic structures of crystals. Q.J.G.S., vol. XIV. 1858, p. 476. 

 S. ALLPOET. On the Pitchstones of Arran. G.M., vol. IX., 1872, p. 4. 



(2) Figs. 8, 9 and 10 are taken from Dr. Sorby's paper. 



(3) F. ZIEKEL. Microscopical Petrography of the 40th Parallel. 1876, p. 226. 



