CnAV. XXXVIT. ] 
GO A. 
983 
nation and raetasomatic replacement by means of ferruginous solutions 
acting indiscriminately on the DMrwar rocks. The source of the iron 
is probably to be looked for in the abundant ferruginous rocks found 
amongst the Dharwar rocks. In some cases the formation of the laterite 
has probably taken place by the mere re-arrangement on the spot of 
the iron-ores of the Dharwars. But in those cases where the laterite 
has formed on the top of nou-ferruginous rocks, such as quartzites, the 
iron must have been brought in solution, at least from the nearest 
occurrence of ferruginous rock. In some cases the laterite has forra^ed 
on the top of comparatively fresh rock, and in other cases on altered 
rock. Sometimes this altered rock seems to have been the quartzite 
partly replaced by manganese-ore referred to above. In such cases, at 
least a portion of the manganese in the overlying laterite must be taken 
as representing the manganese contained in the quartzite. It may 
remain in the laterite either in the form in which it was contained in 
the quartzite, or it may have been dissolved and re-deposited, or re- 
arranged by segregative tendencies. In other cases the manganese in 
the laterite may be brought in solution together with the iron at the 
time of the formation of the laterite. In either case the question 
arises — Where has the manganese come from that is now found in the 
laterite, and the underlying decomposed quartzite or other rock ? The 
answer must be the same as in the case of most of the ores formed 
superficially on the outcrops of Dharwar rocks in other parts of India :— 
The source is not evident, but probably the manganese was originally 
distributed in small quantities through the various rocks— especially 
iron-ores and phyllites — of the Dharwar series, and has subsequently 
been concentrated under the influence of percolating waters and de- 
posited in the form of manganese -ores in the situations where it is now 
found. 
The commonest of the Goa ores seems to be pyrolusite ; but a 
fair amount of psilomelane and wad are also 
Nature and quality of <■ n m n 
the ores lound. ihe ores are usually very cavernous 
and often mixed with iron-ores, such as 
ochre and limonite ; and, as might be anticipated from their mode 
of origin, they often contain residual patches of quartzite and other 
rocks. # 
