236 
MANGANESE DEPOSITS OF INDIA : GEOLOGY. [ PaRT II : 
exposed at the surface wherever the denuding agents of Nature have 
removed the once overlying fossiliferous and Purana rocks. Since the 
total thickness of the Archaean rocks is probably considerably greater 
than that of the whole of the post- Archaean rocks, by far the larger por- 
tion of the earth's crust must be constituted by these ancient rocks. 
As might be expected, the Archaean group is of extremely complex 
constitution, and may be supposed to be composed 
The oldest gneisses. ^ « . . . '■ 
01 nve mam divisions : — 
1. The original crust of the globe, by which is meant the portion of 
it that first crystallized out from the molten condition during the secular 
cooling of the earth. 
2. The sediments formed by the denudation and re-deposition of 
portions of this crust, the denuding and depositing processes being then, 
however, vastly different to what we now observe, and probably at least 
as much of a chemical as of a mechanical nature. 
3. The igneous rocks intruded from the still molten interior of the 
earth into the solid or semi-solid crust composed of 1 and 2. These 
intrusions were, no doubt, taking place during the whole of the time of 
formation of 1 and 2. 
Sedimentation and igneous intrusion and extrusion must have pro- 
gressed side by side, accompanied by a constant crumpling up of the rocks 
thus formed, with a consequent metamorphism of the whole of them into 
the gneissic condition. All the gneisses thus formed have been so folded 
together that to separate them one from another is now in all probability 
everywhere impossible ; especially as these earliest sediments when 
rendered gneissic must be practically indistinguishable from the rocks 
from which their material was derived. Consequently all these rocks 
can be grouped together as the oldest gneisses. 
4. Gradually the meteoric conditions must have begun to approach 
those we now experience, so that true mechanical 
The schistose gueis- sediments were formed. The sediments formed 
sesand the Dharwars. ... 
were then no longer necessarily of a composition 
approaching that of the original igneous rocks, but composed 
of conglomerates, sands, clays, and Umestones. There must have been 
a period of comparative tectonic quiescence during which a vast thickness 
of these sediments was able to accumulate, accompanied by the extrusion 
in some areas of contemporaneous basic lava-flows. 
Both sediments and lava-flows were then involved in another great 
series of tectonic disturbances and more or less deeply folded in with the 
