On the Fissure-Inclusions. 113 



fluid-inclusions, until another bundle of fibrolite is reached 

 on the further side of the grain, and the rock-fissure re-ap- 

 pears as a minute dark crevice cutting across the needles as 

 before. 



All these occurrences are illustrated in the drawing made 

 under a 4-10 inch objective, by a Camera Lucida, presented in 

 Plate X. The quartz-field in the center, shot through 

 with but a few blades of fibrolite, is traversed by several anas- 

 tomosing fissures, as lines of inclusions, which at one end are 

 cut off by a composite scale of muscovite and opaque biotite, and 

 at the other are transformed into the veins of fibrolite-materi- 

 al. The occurrence of the fluid-inclusions in planes, which in 

 most cases lie obliquely to that of the thin section, is easily 

 studied by deeper focussing, but could not of course be rep- 

 resented in the drawing; to this fact is due the occasional ap- 

 pearance in the latter of superposition of the inclusions upon 

 a fibrolite needle, or the slight projection of a needle across 

 the fissure within the quartz-field. 



All these phenomena are interesting evidences of the micro- 

 scopic results of the internal and gradual movements within 

 the mother-rock in the course of folding. The mass has been 

 repeatedly seamed by minute fissures, yielding the plasticity 

 long-recognized in rock-masses of apparently the greatest 

 rigidity, and repeatedly re-cemented by siliceous films depos- 

 ited out of the concentrated and heated solutions which sat- 

 urated the rock. The process was closely akin to that of the 

 repeated fissuring and re-cementation in the ice-grains of a 

 glacier. The huge veins, selvage, and slicken sides which are 

 the familiar macroscopic results attendant upon such move- 

 ments, here show in some respects their microscopic counter- 

 parts. By the slow grinding action, the greatest width of the 

 fissure was attained and a fragmentary vein produced, where 

 the softer material (fibrolite) occurred along its course : some 

 needles being simply torn out or pulled apart, but not broken, 

 where the rending force diminished, near the thinning out of 

 the fissure. 



By the fortunate co-incidence of such fissures of sufficient 

 length, passing across both quartz and a fibrous mineral like 

 fibrolite, with their subsequent partial occupation by dissolved 

 silica (a common cementing material in many rocks), the rock 

 under investigation apparently presents clear evidence that, in 



