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patches which appear after rain on walls and houses caused by species of 

 Lyngbya and Protococcus. The most remarkable case, however, of colouration 

 by such a cause is that of the Pedras ncgras (Black rocks), in Angola, West 

 Africa. These had been noticed by the old Portuguese travellers centuries back, 

 and described by them. When Dr. Welwitsch, the celebrated African traveller, 

 visited, a few years back, the district of Pungo Andongo, in which these hills 

 occur, he, however, saw them to be of a pale yellowish, grey colour, rather 

 darker at the summits. This was in October. In the next month heavy rains 

 occurred, and a few days after, Dr. Welwitsch, on visiting the district again, ob- 

 served the hills to be black from the summit to the base. On proceeding to the 

 base, an examination quickly showed the colouration to be due to a filamentous 

 Alga of the genus Scytomma, which covered miles of the hill-sides. It appeared 

 to take its origin from pools at the top of the hills. When the dry weather set 

 in, it dried up and became white, and soon peeled off, leaving the hills their 

 natural colour (applause). 



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