114 American Quarterly Microscopical Journal. 



this case at least, the extension of the fluid-inclusions in 

 planes indicates for them a fissure-origin, secondary to, and // 

 may be far later than, the genesis of the rock. It is also easy 

 to understand how readily, as such a capillar}- fissure is grad- 

 ually filled up, portions of its area may be entirely occupied by 

 gas, steam, or a liquid, and resist and prevent the deposition 

 of silica, until a barrier had been formed. The constant ten- 

 dency to cr)-stallization within this siliceous deposit produced 

 the abundant cavities of cr}-stalline form, frequently modified 

 by partial re-solution and erosion of angles. The accidental 

 enclosure in each case of gas or vapor or liquid under heavy 

 pressure, with an additional introduction or escape of any one 

 of these before the barrier became complete and the cavity 

 hermetically sealed, and with the subsequent contraction on 

 cooling, afforded all the conditions apparently necessary- for the 

 plane arrangement of the inclusions as now found. Thus, 

 too, they occur completeh* or partially filled with either gas or 

 liquid- but with a general correspondence in regard to con- 

 tents, in contiguous groups of inclusions. 



It may be further stated that just such parallel lines of 

 fluid inclusions cross the quartz-grains in many other gneisses 

 and granites of this countr)-. 



From these facts we may conclude that in many rocks those 

 gj-oups of fluid-inclusions, which are marked by a plane 

 arrangement, are of secondan,' origin, resulting from a late 

 injection, as suggested by Vogelsang — not exactly of cleav- 

 age cracks, however, but of rock fissures. In such cases they 

 can throw light only on the conditions of metamorphism in 

 a rock, rather than on those of its origin. Doubtless in the 

 same rock, or even in the same quartz-grain, fluid-inclusions 

 of both kinds may co-exist, and a careful distinction between 

 them, whenever possible, is requisite in investigations like 

 those of Sorby already alluded to. 



The great variations in relative volumes of bubble and liquid 

 and fluid contents in general, in closely contiguous cavities, 

 often remarked by many observers, may be in part due to 

 this difference in their origin, and may yet serv^e as a means 

 of distinction in certain cases : e. g. -where fissure or second- 

 ary inclusions may be distinguished from the primarj' by the 

 co-incidence of certain characteristics (the same contents, or 

 the same volume-relationship of bubble and liquid) along cer- 



