Chap. XI. ] 
GENERAL. 
small proportion of the material has been derived from the igneous rocks 
of post- Archaean age. 
Let us take the case of the element manganese. As has already been 
mentioned (page 18), the earth's crust contains on 
The ultimate source the average about 0*10 per cent, of manganese 
ores*^^ protoxide. Most of this was originally dis- 
tributed in insignificant proportions in the various 
ferro-magnesian silicates, such as garnets, amphiboles, pyroxenes, and 
micas, contained in the Archaean igneous rocks. But igneous rocks have 
also been found in which highly manganiferous minerals form an im- 
portant part of the rock : thus in the kodurite series of India manganese- 
garnets and manganese-pyroxenes are found in abundance ; whilst the 
albite-pegmatite of Branch ville in Connecticut, United States of America, 
contains numerous highly manganesian phosphates. The operations of 
Nature at the surface of the earth on the great variety of rocks exposed 
to her action are nearly always such as to produce, by either mechanical 
or chemical means, a concentration at particular spots of particular 
constituents of the rock operated upon. This has led to the concentration 
at particular spots, in the form of workable deposits of manganese-ore, 
of a certain proportion of the once widely disseminated element manga- 
nese ; and it is now almost impossible to mention a country in which 
some evidence of the presence of manganese, if not as workable deposits 
of ore, at least as nodules, impregnations, coatings, or black stains, 
cannot be found. 
It follows from what goes before that workable deposits of manganese* 
Manganese-ore de- might be found in rocks of any age whatsoever, 
posits found in rocks all thai is required being a favourable combination 
of any age. circumstances, either mechanical or chemical, 
enabling the requisite concentration to be effected. Such is found to 
be the case. Thus the deposits of Brazil and most of those of India 
lie in the Archaean rocks, those of Arkansas and the Appalachian region 
of the United States chiefly in the Palaeozoic, those of Carniola in the 
Trias, and those of the Caucasus in the Eocene. 
Although most of the workable manganese-ore deposits of India occur 
^ ., ^ , in rocks of Archaean age (those in laterite forming 
Distribution of the ^, ^. , ^ n • , 
nianganese-ore« and the exception), yet manganese-ores and mmerals 
mii^erais o^f India ac- liave been recorded from a variety of formation.s 
coi ing o or a ions, rpj^^gg forth in the following table, the oldest 
rocks being placed first and the youngest last. It must be noted 
