A detailed magnetic survey conducted in the Waltair area indicated that the charnockite rocks are more magnetic than the country rocks, khondalites and leptynites. The magnetic data suggest the possible occurrence of some unexposed charnockites. Some inferences regarding the depth of occurrence, width, and the direction of magnetization of the causative bodies have been drawn.Une survie magnttique dttaillte effectute dans la rtgion de Waltair indique que les charnockites sont plus magnttiques que les roches de I'endroit i.e. des kondalites et des leptynites. Le relevt magnttique suggkre une occurrence possible de charnockites non expostes. Des inftrences sont faites quant B la profondeur de l'occurrence, sa largeur, et la direction de sa magnttisation.[Traduit par le journal] Introduction this region are confined mostly to plains. This A regional magnetic survey conducted pre-is an exception since they are generally assoviously around Visakhapatnam by Rae (1958) ciated with uplifted blocks of the Indian Peninrevealed the high magnetic response of char-sula (Pichamuthu 1951;Radhakrishna 1968). nockites, which indicated the possibility of The charnockites intrude the country rock, and locating subsurface charnockites by further de-the khondalites (Rae 1947; Chari7 persona1 tailed investigations. Moreover, it is felt that a communication), and have undergone Post magsystematic study of the magnetic properties of matic changes such as albitisation. They have charnockites both in the field and in the labors-also formed hybrid gneisses with the khondatory may reveal something about the nature lites and in some places with other gneisses. of the mineral or minerals contributing to the The typical khondalite is a rock of the granumagnetism of the rocks. In order to study the litic facies derived from high alumina clays, magnetic behaviour of ,-harnockites, a magnetic which are also fairly rich in iron. Leptynites survey was conducted in Waltair (near 170 N are metamorphosed products of khondalites and 83" E ) , where all the four types of char-and these also have been intruded by the nockites (acid to ultrabasic) occur, associated charnockites. Laterites form the disintegrated with khondalites and leptynites; the country products of the khondalites. The geology of rocks. Preliminary results of this survey were the area is s h~w n in Fig. 1 . reported earlier by the author (Aravamadhu

A gravity ‘low’ with an amplitude of some 70 mgal over the east Cuddapah basin of Precambrian (Proterozoic) age has been mapped which extends for about 400 km paralleling the eastern boundary of the basin with an average width of 60 km. This near parallelism between the boundary of the basin and gravity anomaly contours suggests a strong correlation between gravity and geology of the Cuddapah basin. Direct calculations based on the techniques of Bott & Smith (1958) and Skeels (1963) suggest the anomaly source to be within the crust. In view of these direct calculations and some known granitic intrusions in the northeastern part of the basin, it is suggested that the anomaly is caused by massive granitic intrusions into the heavy rocks of Cuddapah system on the west and Dharwar system on the east. Although granitic intrusions are known only at a few places in the northeast, the extension of the gravity anomaly suggests that the granitic activity may be widespread and may occur all along the eastern Cuddapah basin. An average thickness of 20 km with a density differential of 0.1 g/cm 3 is needed to account for the observed variations in the isostatic anomaly. It is suggested that the granites with which mineralization in the Cuddapah basin is presumed to be associated may be cupolas of this inferred granite mass. For the inferred east Cuddapah basin crustal column to be in isostatic equilibrium, the presence of about 16-5 km material of 3.01 g/cm 3 density between the mantle and the inferred granite or crustal thinning of the order of 6.5 km is required. This leads to a suggestion that in anomalous regions due consideration should be given to the compensating effect of anomalous masses that may lie below the sea level in addition to the usually adopted method of considering only topographic masses in computing isostatic effects. On the basis of the comparison of the east Cuddapah basin crustal column having 20 km of inferred granitic mass with a standard continental crustal column (e. g. that due to Worzel & Shurbet 1955), it is concluded that granitic intrusions have locally reduced the density of an originally heavier upper crust.