EXPLANATION OF PLATE 13. 
ir^OTOMICaOGRAPHS. 
Fig. 1, — Shows innumerable black prisms of a bia- k manganese mineral, scat- 
tered thron^h a matrix of t:ne-grained m<j aie quartz. 
Fig. 2. — The apparent ca\ity with inwardly proiectine needles is really filled 
with qu rtz mosa'c, as can b- seen when the nicols are crossed. 
Fig. 3. — Quartz — white ; psilomelane — black. 
Fig. 4. — The quartz is chai white, whilst the feLpar has it^^ cleavage picked out 
by thin filpis of manganese oxide black i deposited alcmg the 
cleavage planea. 
