.

PRE-TERTIARY VOLCANIC ROCKS.

CHAPTER I.

THE DECCAN TRAP-SERIES OF INDIA.

The great outpourings of augitic lava of Tertiary and recent times which
we have been considering appear to have been anticipated in several
parts of the world, more especially in Peninsular India and in Africa,
and it is desirable that we should devote a few pages to the description
of these remarkable volcanic formations, as they resemble, both in their
mode of occurrence and general structure, some of the great lava-floods
of a more recent period we have been considering. Of the districts to be
described, the first which claims our notice is the Deccan.

(_a._) _Extent of the Volcanic Plateau._--The volcanic plateau of the
Deccan stretches from the borders of the Western Ghats and the sea-coast
near Bombay inland to Amarantak, at the head of the Narbudda River
(long. 82 deg. E.), and from Belgaum (lat. 15 deg. 31' N.) to near Goona (lat.
24 deg. 30'). The vast area thus circumscribed is far from representing the
original extent of the tract overspread by the lava-floods, as outlying
fragments of these lavas are found as far east as long. 84 deg. E. in one
direction, and at Kattiwar and Cutch in another. The present area,
however, is estimated to be not less than 200,000 square miles.[1]

(_b._) _Nature and Thickness of the Lava-flows._--This tract is
overspread almost continuously by sheets of basaltic lava, with
occasional bands of fresh-water strata containing numerous shells,
figured and described by Hislop, and believed by him to be of Lower
Eocene age. The lava-sheets vary considerably in character, ranging from
finest compact basalt to coarsely crystalline dolerite, in which olivine
is abundant. The columnar structure is not prevalent, the rock being
either amorphous, or weathering into concentric shells. Volcanic ash, or
bole, is frequently found separating the different lava-flows; and in
the upper amygdaloidal sheets numerous secondary minerals are found,
such as quartz, agate and jasper, stilbite and chlorite. The total
thickness of the whole series, where complete, is about 6,000 feet,
divided as follows:

1. Upper trap; with ash and inter-trappean beds 1,500 feet
2. Middle trap; sheets of basalt and ash 4,000 "
3. Lower trap; basalt with inter-trappean beds 500 "
--------
6,000 "