Page:Madras District Gazetteers - Anantapur.pdf/29

west for close upon 22 miles, beyond which it has not been mapped, the survey being left unfinished. It probably dies out a few miles further south. This Dharwar rock is younger than the Archean formations upon which it rests, and not, as was at one time supposed, an older series which has protruded through them, After it was deposited a period of immense denudation supervened which re- moved almost all of it. The great interest of the Dharwar rock in other areas lies in the metals ( iron, manganese, gold, etc. ) which it contains, but in Anantapur this small band is apparently destitute of minerals of importance.

The principal rock varieties to be noted in the band are horn- blendic schist and dark trappoids. To the east and south-east of the point where it crosses the Pennér are a number of outliers of Dharwars which represent so many parts of foldings which have been caught down into small faults in the Archzean rocks.

The alluvial and subaerial formations met with are of little importance, the sand-dunes on the banks of the Pennér near Tadpatri and the travertine rocks of the Kona-Uppalapddu vailey, both of which are referred to in the accounts of these places in Chapter XV below, being perhaps the only cases of interest.

The Archean gneissose rocks of the district show considerable variety. They are for the most part granites, In the northern part of the district, north of the Madras Railway, porphyritic syenitic rock forms a number of bouldery hills, and the Ndgasamudram range already referred to is a band of the same kind. Similar rock occurs frequently about the surveyed part of the district but micaccous granite is far more common. These are probably mere local variations of one and the same magma. As a rule they are much alike in general appearance and the mineral difference appears only on close examination. A noteworthy variety is the very handsome red rock which forms the northern half of the Nagasamudram line of hills. The granitic rocks rise into several fine bold peaks such as Gampamalla, north of the Singanamaila tank, the Kalyandrug group and the hills south and south-east of Dharmavaram. The unsurvey- ed southern half of the district around Penukonda is also well-known for its bold and high granite hills,

A remarkable feature of the Archzean region is the vast number of dioritic trap dykes which traverse it. Some of these may be traced for long distances, often for 40 or 50 miles, with but very trifling breaks. A noticeable instance is the dyke which starts a few miles north-west of Penukonda town and runs almost uninter- ruptedly across the taluk to Bukkapatnam, where it partly flanks,