386 W. CAMPBELL SMITH. 



Several sections were cut parallel to a weathered surface, 

 so that their positions with regard to the centres and edges of 

 groups of rays could be determined ; another was cut in a 

 plane at right angles to these, and two others were cut from 

 the body of the rock about one inch from the nearest 

 weathei»ed surface. An examination of these sections quickly 

 revealed the cause of the macroscopic effects. 



The first section, cut so that one of the centres appears on 

 the edge of the slide, shows the labradorite as long thin 

 laths diverging from points grouped around the centre. As 

 the rays are traced outward from the centre fresh groups of 

 felspars are seen to start from various points along the line, 

 each group spreading out like a fan, so that the result is a 

 roughly radial structure (PI. XXIX, tig. 1). No section 

 could be obtained showing that there was any definite nucleus 

 at the centre of a group. 



In a section cut to show one of the boundaries between two 

 groups a sharp line can be seen with the naked eye corre- 

 sponding to the straight line on the weathered surface. 

 Under the microscope this line is cleai-ly shown to be caused 

 by the confluence of two streams of felspar-laths diverging 

 from the opposite centimes. The line is not perfectly straight 

 but is slightly undulating (PL XXIX, fig. 2) ; the felspars 

 fi'om one centre tend to push in between those coming in the 

 opposite direction, which explains Avhy the cracks formed on 

 cooling run parallel to the rays rather than along these lines 

 of junction. 



Sections showing part of a space apparently free from 

 radial structure reveal a rather confused mass of short felspar 

 crystals arranged in little bundles with here and there a few 

 almost square sections. Such sections are cut almost at right 

 angles to one of the radial groups of felspars, which, fanning 

 out from numerous points, produce a curiously confused 

 appearance. 



Similar structui'es to those described above wei-e shown by 

 sections taken in different planes and in different parts of the 

 rock. 



