116 ROCK INCRUSTATIONS. 



EXPLANATION OF FIGURES. 

 Plate I 



1. Photomicrograph of rock-section (quartz) ground at right angles 



to alga-incrusted surface, showing thickness and transverse 

 section of the alga ( Hildenbrandtia). The parallel series of 

 minute, super-imposed cells arranged transversely to the rock 

 surface are evident in parts of the picture, x 250 diameters. 



2. Photomicrograph of a flat, expansive fragment of the alga ( Hilden- 



hrandtia) scraped from rock surface showing the ineshwork of 

 cells. X 070 diameters. 



3. Photomicrograph of a flat, expansive fragment of the black coating 



scraped from rock surface and rendered transparent by the 

 action of hydrochloric acid. Outlines and impressions of the 

 alga ( Hildenhrandtia) cells are shown in the picture, x G70 

 diameters. 



4. Photomicrograph of rock section (quartz) ground at right angles 



to incrusted surface, showing the thickness of the black, opaque 

 coating whose origin is attributed to the incrusting alga, x 250 

 diameters. 



Note. — In comparing figures 2 and 3, the slightly larger size of some 

 of the cells in figure .3 is accounted for by the fact that the cell 

 contents and not the cell walls are alone prominent in figure 2, 

 whilst in figure 3 the impressions of the cell walls are chiefly 

 conspicuous 



