112 American Quarterly Microscopical Journal. 



yellow ; associated and often contiguous scales of colorless 

 muscovite; pinkish grains of garnet, dotted with colorless 

 spherules of quartz; opaque and black granules of magnetite, 

 and brownish-red films of iron-ochre. It was further observed 

 in every thin section (most satisfactorily under a \ inch objec- 

 tive) that the rock is traversed by very numerous and exceed- 

 ingly minute fissures, partly m planes which are approximate- 

 ly parallel, at least within the area of the thin section, and part- 

 ly as branching cracks in an irregular network. The' courses 

 wind slightly, but irregularly, showing little or no relationship 

 to the cleavage-planes of the minerals which they may traverse. 

 In crossing the bundles of fibrolite needles, such a fissure is 

 represented by a minute dark line, by interruption of the con- 

 tinuity of that material, with a somewhat jagged border, the 

 fibers or blades of fibrolite being continued on either side, with 

 splintered ends but generally without dislocation or fault. 

 The fissures sometimes appear to be empty in places, or oc- 

 cupied and darkened by minute ochreous particles. More com- 

 monly, however, it appears as a vein of laminated structure, 

 consisting of a finely granulated (X925), yellowish material 

 (crushed fibrolite) on each side, next the walls, but in the cen- 

 ter of a delicately laminated and colorless substance, appar- 

 ently quartz, with a very thin and perhaps empty middle 

 suture. The fluid-inclusions or cavities are extremely rare in 

 the vein within a fibrolite-field, and always exactly along the 

 central suture; they occur only close to the entrance to a 

 quartz-field, or enclosed in the occasional quartz-granules 

 within the interstices of the fibers. Some of these veins thin 

 out and disappear within the fibrolite-fields, and near their 

 terminations are crossed by short blades, with fractured ends. 

 The width of the central laminated band — the true width of 

 the original fissure — varies from 0.00007 to 0.00014 mm., and 

 that of the entire vein, including the fragmentary bands, up to 

 about 0.00135 mm. 



Within a quartz-field, on the other hand, where the same 

 fissure crosses that material within the interstices of the fibro- 

 lite-network, or, more distinctly, where it crosses an adjacent 

 -grain of quartz, its width is confined to that of the central band 

 above described, sometimes still continued in the same capil- 

 lary crevice, or represented interruptedly by short sections of 

 crevice with clear quaitz between, or very often by a line of 



