No detailed account of the geology of the district has yet been published.1[1] CHAP. I. Geology. The fundamental rocks are all gneisses and plutonic igneous rocks of the archtean group. They outcrop in lines running mainly from north-east to south-west, which direction determines that of the chief plateaus and minor hill ranges. The district may be divided geologically into four parallel zones; namely, (i) the 2,000 feet plateau in the north-west, composed of the older sub-group of archean gneisses, namely biotite and horn-blende mixed gneiss with layers of steatite, some younger diabase dykes (almost the only dykes in all the district) and a few outliers of Cuddapah quartzites with some crystalline limestone,
(ii) the north-west portion of the 3,000 feet plateau, made up of bands of the younger arcliaean sub-group of khondalite and intrusive bands of charnockite,
(iii) the south-east part of the same plateau, consisting of more khondalite (with local beds of iron and manganese ore and crystalline limestone) and bands of charnockite again and coarse porphyritic biotite gneissose granite, and
(iv) a coastward low-level zone containing minor ridges composed almost exclusively of yet more khondalite with a few bands of charnockite and gneissose granite.
In these last rocks occur the manganese deposits mentioned below. The most obvious characteristic of the gneisses is the number of brown or purple-brown iron garnets which are scattered through them. White quartzose gneiss streaks the surface of parts of the country, especially between Vizagapatam and Vizianagram, with conspicuous reefs and ridges, but it does not occur in true veins and is not auriferous.
The surface rocks include horizontal plateaus of high-level pisolitic laterite some 80 feet thick at an elevation of from 3,500 to 4,000 feet above the sea, chiefly to the north of Koraput and spreading out in the direction of the Kdlahandi State. This laterite, which has been thought to be a sedimentary deposit laid down in water, is limited to a fairly constant level, and surrounds the hills like a belt of shore through which the bare rocks, which were perhaps islands in the lateritic age, raise themselves. It contains much hydrated alumina and may possibly prove of value as an ore of aluminium. Other recent deposits include the younger alluvium of the plains, an older red lateritic loam and the blown sands of the coast, both of which latter are very noticeable at Waltair.
- ↑ 1 The following pablications of the Geological Survey of India refer to the subject:—Records, xix, pt. 3; xxxi, pt. 1; xxxii, pt. 2; xxxiiii pt. 2; and the General Reports for 1899-1900 and 1902-03.
