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usually very minute, and therefore visible only when thin sections of the rocks 

 are examined under the microscope. The largest are usually about "006 mm. in 

 diameter ; the smallest are visible only as specks when viewed with the 

 highest powers of the microscope. They are most abundant in quartz, but 

 occur also in felspar, nepheline, elaeolite, leucite, augite, chlorite, olivine, topaz, 

 cordierite, vesuvian, beryl, spinel, sapphire, calcite, gypsum, fluor-spar, 

 rock-salt, cassiterite and zinc-blende. If we consider the distribution of 

 fluid inclusions in the different classes of rocks, we are struck by the fact that 

 they are especially characteristic of the plutonic rocks, such as gabbro, diorite 

 and granite, and the crystalline schists. They are rare or absent in rocks of 

 the volcanic group. Speaking generally we may say that fluid and glass 

 inclusions bear a sort of inverse relation to each other so far as distribution is 

 concerned. Where glass-inclusions are common fluid inclusions are either 

 very rare or altogether absent. We do occasionally find glass- and stone- 

 inclusions in the minerals of certain granites and fluid-inclusions in those of 

 volcanic rocks, as for instance in the olivine and leucite of certain lava 

 streams ; but the rarity of these occurrences only serves to emphasize the 

 general truth of the above statement, and it must also be remembered that in 

 these exceptional cases the minerals in question have probably been developed 

 before the actual eruption of the lava. 



The relative proportion of the fluid- and glass-inclusions has been 

 supposed by some petrographers to stand in relation to geological age ; 

 the fluid inclusions being regarded as especially characteristic of the 

 older and the glass - inclusions of the younger rocks. (1) The supposed 

 fact has been explained by reference to changes in the physical 

 condition of the planet during the period of time represented by our 

 geological records. That such changes have occurred, may be admitted ; but 

 that we have any evidence of them in the fact here referred to appears to the 

 present writer extremely improbable. The result is obtained by comparing 

 plutonic with volcanic rocks ; or, in other words by comparing the rocks which 

 have consolidated at great depths and under great pressure with the surface 

 products of volcanic action. If we compare the plutonic rocks of different 

 periods with each other, and the volcanic rocks of the different periods with 

 each other, the supposed fact disappears. Fluid inclusions are equally 

 abundant in the plutonic rocks of all ages, and glass-inclusions are 

 equally abundant in the volcanic rocks of all ages. Considerable light is 

 thrown on this subject by a paper by Messrs. HAGUE & IDDINGS on " The 

 Development of Crystallisation in the Igneous Rocks of the Washoe 

 district " (2 ' The authors show that the diabase and diorite of this region are 

 the granular representatives of the porphyritic augite- and hornblende- 

 andesites, and shade into them in a perfectly gradual manner. Glass- 

 inclusions occur in the latter, fluid-inclusions in the former rocks. 



Fluid inclusions vary considerably in form. They may be spherical, 

 elliptical, irregular or bounded by planes corresponding to the faces of the 

 containing crystal. 



(1) C. FRENZEL. Ueber die Abhangigkeit der mineralogischer Zusamniensetzung und 

 Structur der Massengesteine vom Geologischeu Alter. Zeit. f.d. Gess. Naturvv. Bd. LV., 1882. p. 1. 



(2) Bulletin of the U.S. Geological survey. No. 17. 



