CAUSES OF DECAY 387 



structlble rocks, as the siliceous conglomerates of the Shaw- 

 angunk mountains, and where there are no evidences of 

 surface decay, are yet covered with lichens. It Is possible 

 that there Is no effect here."^* It Is well known that certain 

 rocks are suited to certain species of mosses, and a more 

 vigorous growth is a mark of some kinds of stone. The 

 green confervae, so often seen in damp weather upon more 

 porous stones, are Indicative of moisture and of greater ab- 

 sorptive capacity. As it disappears with the season, and 

 has almost no hold on the stone. Its effect must be slight. 

 It is rather an Index of a porosity, and of a consequent ten- 

 dency to decay, through the action of water and frost. 

 Where the mosses thrive there will be some accumulation 

 of dust and a lodging place for spores and seeds of plants, 

 whose rootlets can do harm in the case of Inferior stone. 

 And this growth in turn favors the further accumulation of 

 material In which it may continue to thrive. In nature the 

 ledges of softer and more porous stone exhibit the growth 

 of confervae, whereas the lichens thrive upon the harder 

 and more compact rocks.f 



The fungi, from their more penetrating power, may do 

 Injury, as in their causation of decay and rots in the case of 

 wood. 



II. Chemical Agents 



The chemical agents are the following: 



1. The solvent action of waters carrying carbonic, sul- 

 phuric, nitric, hydrochloric and organic acids. 



2. The oxidizing and deoxidizing action of the air, In the 

 presence of moisture. 



* " When firmly established, lichens seem to exercise a protective influence against 

 the ordinary causes of decomposition of the stone upon which they grow." Gwilt, 

 Encyclopedia of Architecture, London, 1867, p. 456. 



f " None of the softer rapidly decaying rocks produce this vegetation." Prof. James 

 Hall, 39tli Annual Report New York State Museum, Albany, 1886, p. 218. 



