. .KOLOGY S05 



as might have been graven by the point of a quart/ 

 crystal, others closely adjacent being blunt and coarse, 

 as if produced by the edge of an angular block of stone. 

 Moreover, the striated surfaces arc undulating and 

 cd, the strut descendr and rising out of 



these inequalities. Here and there, the grooving! may 

 be seen mounting up a sloping boas of rock, but if the 

 inclination of the rock becomes steep, the markings 

 diverge on cither M,<. . >t it. On a vertical crag, they may 

 be found running horuontally along its face. Now a 

 nsided surface presents, in many respects, a con- 

 trast to these features. It is almost always coated with 

 some mineral glaze or incrustation, such as hematite, 

 calcite, chlorite, or quartz, which may have taken a cast of 

 the strut- on the rock, or may subsequently have 1* 

 self striated I n innumerable instances, the slickenside is 

 not confined to one surface, but may be detected in 

 successive planes inside the rock, showing it to be an 

 internal condition of the mass due to the shifting and 

 grinding together of its parts, and not to a mere super- 

 ficial agent like moving ice. The striae of slickensides 

 are close set, parallel, and tolerably equal in breadth and 

 depth, and tlu-y lie on flat surfaces which do not un- 

 dulate in the manner so characteristic of glaciated rocks. 

 It is important to take with the compass the direction 

 of the glacial groovings and stria: on rocks. If possible 

 the observer should at the same time determine from 

 M quarter the ice has moved. This may often be 

 done by noting in what direction little prominences and 

 the edges of angular projections are rounded off, and 

 to which side the still rough and unstriated portions look. 



