112 ROCK INCRUSTATIONS. 



in hydrochloric acid, forming a pale brown solution which 

 yielded a precipitate of ferric hydrate on the addition of 

 ammonium hydroxide, indicating the presence of iron 

 in the coating. A minute quantity of bleached residue, 

 which appeared to be structureless, was recovered in some 

 cases. 



Rarely the coating completely envelopes the rocks. 

 Generally, there is a small or great part of the surface of 

 the rock unincrusted, and the margin of the coating is mostly 

 verj' irregular in outline. Sometimes small uncoated 

 patches appear on a coated surface. 



Field observations first suggested that the black 

 coatings may be the altered remains of an incrusting alga. 

 This conclusion is supported by the following facts : 

 (a) correspondence in distribution : (b) comparability of 

 thickness ; (c) the presence of the cellular structure of the 

 thallus of the alga in 50 per cent, of the examples of the 

 black coating rendered transparent by hydrochloric acid. 



The fine, red-coloured, incrusting alga grows on the 

 rocks in the streams flowing through the rain forests. A 

 specimen of it was sent to Mr. J. H. Maiden, Government 

 Botanist of New South Wales, who replied : — " Mr. Lucas 

 (Honorary Algologist to the National Herbarium, Sydney), 

 thinks that the plant collected by Mr. Francis is a species 

 of Hildenbrandtia, a red incrusting alga. The species are 

 common on rocks on the Coast." It consists of a very 

 thin coating, varying in thickness from about 5// to Hljbi, 

 is very closely and firmly adherent to the rock surface, and 

 is composed of a variable number of super-imposed cells 

 (see Fig. 1, Plate I.). When small pieces are scraped 

 from the rock and examined under the microscope, they 

 present the appearance of a meshwork of minute cells 

 which are angular or rounded, and vary from 3/j, to 8^ 

 in breadth. (See Fig. 2, Plate I). The variation in 

 the thickness of the plant is proportionate to the number 

 of cells which are super- imposed in any particular portion 

 of the thallus (which constitutes the plant). The very thin 

 portions of the thallus may be only one or two cells in 

 thickness, and probably represent the young, growing 

 margins. No special organs of reproduction were observed 



