84 



lines are, apparently, entirely undistorted, being rarely or 

 never indented by adjoining pebbles or layers. Although 

 usually quite homogeneous, a few instances have been observed 

 where the jaspery patches were marked by parallel, narrow 

 bands of lighter and darker red. These coincided in direction 

 with the general lamination of the rock, and yet seemed to be 

 independent of it. Small irregular spots and stripes of the 

 jaspery material are common in the dark-red lenticular layers, 

 and do not appear to be easily separable from an important 

 feature of the latter now to be described. The great majority 

 of these layers have a more or less distinctly concentric 

 structure. They consist of an outer feldspathic portion en- 

 veloping a quartzose nucleus or axis. The distinctness of 

 this concentric division into two layers is inversely proportional 

 to the relative thickness of the sheets ; the axis being usually 

 wholly unobservable in those layers which are thickest relatively 

 to their lengths, appearing as an ill-defined darker band in those 

 of medium flatness, and having a sharply marked boundary 

 only where the thickness does not exceed about one-tenth of 

 the length. There is no minimum limit ; it being possible to 

 detect the axis in layers which are themselves almost too thin 

 to be seen with the naked eye. The axial sheet becomes more 

 quartzose as it becomes more distinct, often reaching the con- 

 dition of vitreous quartz in the finer layers. It is not un- 

 common to find several overlapping laminae of quartz in the 

 same lenticular layer, — a schistose structure in the constituent 

 layers of a schistose rock. These quartz laminae are occa- 

 sionally of quite irregular forms, tinged with red or pink, and 

 indistinguishable from the jaspery patches or stripes. In fact, 

 I believe these two features of the rock are essentially the same, 

 and there can be no doubt, whatever, but that they have both 

 been produced by segregation. 



This schistose petrosilex holds pebbles, and, as elsewhere, 

 they are angular and sharply outlined. They are usually 

 small, although fragments four or five inches across have been 

 observed. There are a few fine-grained granitic and dioritic 



