1 m, ) 
the bark of the oldeft trees, while the broad 
cruftaceous kinds cover young bark, and the 
fmoother forts of ftones and rocks. The 
lichen pafchalis being a perennial of very 
flow growth, many years eiapfe before it's 
crumbling branches fall into the cavities of 
the lava, and there decaying form vegetable 
mould for the nourimment of other plants. 
By attentive obfervation the progrefs, in which 
fuch vegetable mould is formed, may be feen 
on the fmooth and barren rocks upon the fea- 
fhore ; and by a knowledge of the decaying 
plant w T e may know that, which will next 
fucceed. After the by'ffus and feveral fpecies 
of lichen have crumbled into dull, firfl: appear 
other fpecies of lichen, which require a deeper 
foil for their fuftenance. When thefe perifli, 
and have again more thickly covered the 
rocks with mould, various kinds of the moffes 
appear ; in their turn thefe alfo decay, when 
their places are fupplied by other plants, till a 
fufficiency of earth is accumulated to afford 
nourimment to the largeft trees. It has been 
before obferved, that fome of the fpecies of 
lichen are ufed in dying ; one of them, liclirn 
roccella, called the orchel or argel, is brought 
from the Canary iflands, and forms a confi- 
derable 
