Chap. XIV.] dharwars : crystalline limestones. 
299 
suppose that these gneisses have soUdified from igneous fusion. Such 
a theory would, however, need the hypothecation of a magma of very im- 
usual composition, so that it is preferable to suppose that they are meta- 
morphosed sedimentaries. They may have been formed either during 
the Dharwar period of sedimentation, or previously ; but not later, for 
they have been involved in the Dharwar folding. From their genetic 
relationship to the crystalline limestones, and the fact that one variety of 
these limestones is manganiferous and hence probably of Dharwar age — 
although it is not impossible that some of the pre-Dharwar sediments 
were manganiferous — it is probable that the sediments from which 
these gneisses may have been derived were deposited in Dharwar times. 
Since the formation of the crystalline limestones is supposed to have been 
efEected through the agency of alkaline solutions containing carbon 
dioxide, we must suppose that their formation took place during 
or towards the end of the folding of the Dharwars, when the rocks v/ere 
at such a depth that they were at a high temperature and pressure, so that 
the conditions permitted of great molecular mobility. We can suppose 
that under the influence of the great heat and pressure the calcareous sedi- 
ments were converted from impiire limestones into calcareous gneisses with 
the expulsion of large quantities of carbon dioxide. The gas so expelled 
was probably kept in solution at a high temperature and pressure in the 
waters that must be present in all rocks at a great depth ; and as soon as 
the intensity of the pressure and heat conditions somewhat abated these 
waters, charged, as they must have been, with alkalies in addition to 
the carbon dioxide expelled from the calcareous sediments on their 
conversion into gneiss, at once began to attack the recently-formed 
gneisses and reconvert them into limestones. Sufficient carbon dioxide 
would have been available to completely change the gneisses back 
into hmestones had it all remained in the proximity of the gneisses 
untU the conditions became suitable for this reconversion ; but the mere 
fact that the reconversion was not completed shows eifher that a 
considerable portion of it was removed from the scene of action, no 
doubt in the ordinary course of circulation of the waters, before the time 
came for this reversal of the process by which it was set free or that 
if it all remained present the favourable conditions did not last long 
enough for the reconversion to be completed. This explanation will 
account for the fact that it is possible to find these gneisses in every stage 
of alteration into crystalline limestone, better than any other I can think 
of, and seems to be in complete accordance with the facts of occurrence 
and association of these rocks in the field. It has been introduced here 
II E 2 
