384 W. CAMPBELL SMITH. 



We must now turn to the radial niarkino-s brought out on 

 the weathered surfaces. Some idea of the appearance of 



Outline sketch of weathered surface of the lower sill. Natural size. 



these will be obtained from the photograph of a specimen 

 from the upper sill (left-hand side, PI. XXVIII), and one from 

 tlie lower sill (right-hand side). The accompanying text-figure 



