Kin AH AN — On Microscojncnl Stnictiwe of Rocks. 163 



times in lines, but more usually they are irregularly scattered about. 

 A well marked example of the feather arrangement (fig. 22, PI. 8) was 

 observed in one place, which under powers ranging from 300 to 400 

 showed distinctly the connection between this appearance and a system 

 of tubuli ; the feather on the right hand being the oblique longitudinal 

 section of the tubuli, while the feather to the left was formed by re- 

 flected light, and disappears as the power is raised. 



In the quartz of W and B^ some pieces have the labeculae scattered 

 about as in the quartz of W, and in them the tubuli appear to be 

 rare ; but in most of the others the labeculae are in lines or systems, 

 while the tubuli are not uncommon. These tubuli are usually more 

 or less curved (fig. 22, PI. 8), and rarely straight and symmetrical, like 

 the tubuli in the typical quartz of the elvanytes (fig. 23, PL 8). In some 

 of this quartz there are thin hair-like or capillary lines, not visible under 

 a power of 230 ; these are long and short, straight and curved, and 

 cross or branch from one another, a group being shown in fig. 24, PI. 8. 

 Some of these capilloids, under powers ranging from 300 to 400, are 

 seen to be tubuli, while the nature of others, especially some of the 

 long ones, may be different. The labeculte associated with the 

 capilloids, as shown in fig. 24, usually run in more or less regular 

 systems. Some of these labeculae, as previously mentioned in describ- 

 ing the quartz of the elvanyte, are imdoubtedly bubbles, but some of 

 them represent the cross section of the tubuli, the larger ones being 

 sections of the short, thick tubuli, and the minute ones the sections of 

 the capilloids. 



In specimen B* a few of the pieces of quartz give a play instead 

 of a change of colour. 



In specimen B*^ the non-crystalline pieces of quartz, the labeculae are 

 very similar, and similarly arranged to those in the skeleton quartz of 

 specimen B^ Tubuli are usually scarce; those principally observed 

 being capilloid. 



The inlying blebs and secretions of quartz found in the felspars, 

 but principally in the large twin crystals of orthoclase, are usually 

 margined with a prismatic fringe, apparently due to a minute coated 

 structure. They rarely have well defined margins, but usually 

 graduate quietly into the envelope of felspar ; many of them have a 

 centre or nucleus of a foreign mineral. They contain more or less 

 labeculae, some being full of them, and in some were observed the 

 capiUoids and the short thick tubuli. 



