INCLUSIONS IN MINERALS 1075 



these fluid inclusions is manifestly of high importance. Daubree's 

 experiments have shown the enormous mineral -forming powers 

 possessed by greatly heated water, while the presence of liquid carbonic- 

 dioxide testifies to the enormous pressure under which plutonic 

 rocks, such as granite and diorite, have consolidated. 



Inclusions of gaseous matter are also common ; and it is self- 

 evident that the occurrence of one mineral in another is no rarity ; 

 the included mineral being generally the older. To such microscopic 

 inclusions of crystalline bodies is due the remarkable colour of some 

 minerals. In fact, so numerous and so minute are the inclusions in 

 some minerals that even with high powers the minerals appear to 

 be charged with the finest dust. Leucite sometimes affords a good 

 instance of this (fig. 807). Not unfrequently, as with it, the included 

 microlites are so arranged as to outline a crystal of the mineral. 



The foregoing allows us to conclude that an absolutely pure 

 mineral is exceptional. All such mineral bodies contain inclosures 

 of foreign matter which have become entangled during their forma- 

 tion ; when they contain glass-inclusions they have been precipitated 

 out of a mass in the condition of igneous fusion. It follows, therefore, 

 that the presence of amorphous glass, either as 

 a glassy residue or as glass-inclusions, is a 

 frequent characteristic of igneous rocks. Still, 

 the absence of such material does not always 

 demonstrate a non -igneous origin, for plutonic 

 rocks, such as granite, do not possess this feature, 

 having become solid under circumstances which FlG go?. Leucite from 

 brought about complete crystallisation of the Kilimanjaro, East 

 materials. Glass-inclusions are certainly re- Africa, 

 ported by Sigmund l to be present in the quartzes 

 of the granites of the Monte Mulatto, near Predazzo, in South 

 Tyrol, but V. Chrustschoff considers them products of contact- 

 metamorphism. 



We have dealt hitherto more especially with igneous masses, but 

 the sedimentary rocks demand some attention. 



The microscope enables us to recognise to some extent the sources 

 whence the materials composing clastic 2 rocks were derived. For 

 instance, the presence of quartzes containing numerous fluid inclusions 

 (especially those of carbonic dioxide) and hair-like crystals of rutile 

 leads us to conclude they are derived from granites or similar rocks. 

 The cemented material can also be studied and its nature determined. 

 In certain loose sands and sandstones there has sometimes occurred a 

 curious process which the microscope first brought under notice. 

 This is the precipitation on the outer surface of rounded quartz- 

 grains of a greater or less amount of silica, which has been deposited 

 in crystalline continuity with that of the original nuclei (fig. 808). 

 The phenomenon is like that which happens when an irregular frag- 

 ment of a crystal is placed in a concentrated solution of the same 



1 ' Petrographische Studien am Granit von Predazzo,' Jahrb. Jc. k. geol. Reiclis- 

 anstalt, Bd. xxix. 1879, pp. 305-316. 



3 Greek K\a(Trbs = broken. See on this subject T. G. Boiiney, Presidential ad- 

 dress to Section C, Brit. Assoc. Reports (Birmingham), 1886. 



3x2 



