Chap. XVI.] 
GONDITE SERIES : PETROLOGY. 
327 
different composition. Some of these layers, and in many cases nearly all 
of them, consist of rocks containing a high percentage of manganese, and 
are consequently largely composed of manganese-bearing minerals. The 
typical rocks in these manganiferous bands are gondite, spessartite-rock, 
rhodonite-gondite, rhodonite-spessartite-rock, and rhodonite-rock, and the 
manganese-ores, either original or formed by the alteration of the manga- 
nese-silicate-rocks mentioned above. As partings to these manganiferous 
layers are various rocks that either contain only a small amount of 
manganese or are practically free from this element. The commonest of 
these parting layers are quartzites of various colours and texture, their 
characters depending to a large extent on the nature of the impurities 
that the sands from which they were formed contained before they 
suffered metamorphism. Thus the quartzite may be red from the 
presence of ferric oxide, black from the presence of manganese-ores, or Hght 
grey or even white, owing to the comparative purity of the rock. Or 
again, the impurities may have crystallized out as definite minerals giving 
rise to pyroxenic quartzites, spessartiferous quartzites, and so on. In 
other cases the impurities have given rise to various mica-quartz-schists 
and mica-schists, in which the mica often shows unusual types of pleo- 
chroism owing to the presence of small quantities of manganese. Again at 
the periphery of many of the masses of manganiferous rocks various 
interesting rocks are often found that have either been formed by an 
interaction with the manganiferous sediments, or have been produced by 
the metamorphism of sediments that contained considerable quantities 
of manganese, but not enough to make the rock a portion of the ore- 
body. Such rocks often contain manganiferous pyroxenes, garnets, 
micas, and amphiboles. 
In the ore-body the rocks are, as has been mentioned above, usually ar- 
ranged in bands. These bands are often of quite small thickness. Thus 
a rock may consist of layers of gondite and quartz, only ^ an inch to an 
inch thick ; or one layer of gondite or of rhodonite-rock may be as much 
as 1, 2, or even 3, feet thick. On the outcrop, these banded rocks, if they 
have not been largely altered with formation of manganese-ore, often show 
differential weathering. Thus a layer of quartz may stand up leaving the 
alternating layers of gondite occupying slight depressions. 
Owing to their mode of formation, namely by the metamorphism of 
sediments, the masses of manganese-bearing rocks and ores naturally 
appear to be definitely bedded, although, owing to the fact that these 
sediments were deposited in basin-like areas, or because they have been 
