422 MAGNETIC METHOD [Chap. 8 



in northern France, near Aachen, m Hessen, Saxony, and Upper Silesia"' 

 in Germany). 



5. Surveys for miscellaneous ores (chromite, manganese, and the like). 

 As stated previously, direct magnetic prospecting for chromite may be 

 successful if the ores are more magnetic than the basic igneous rocks in 

 which they occur. Reference to magnetic work on chromite is made by 

 Krasulin^^^ in the Urals, by Bagratuni^^ at Geidara (Kurdistan), and by 

 Snelgrove* in Newfoundland. A reference to Lundberg's"* work men- 

 tions magnetic effects of chromite ore in Newfoundland and surveys of 

 chromite-bearing serpentines in Canada. Under favorable conditions, 

 sedimentary manganese ores may be located directly at shallow depth. 

 As shown by a survey made at Nickopol,^^ Russia, the ore, consisting of 

 pyrolusite, psilomelane, polyanite, and wad, occurs at depths of from 20 

 to 40 meters in clay on granite. Magnetic anomalies were small (of the 

 order of 100 7 in Z) and very irregular, corresponding to varjdng composi- 

 tion of the ore and changes in the granitic bedrock surface. 



6. For the location of meteors, the use of the magnetometer is, in most 

 cases, uneconomical. An average iron meteorite, buried several feet deep, 

 is not effective beyond a radius of 5 or 10 feet. Unless its location is known 

 within very close Umits, magnetic prospecting, even with a dip needle or 

 cardan magnetometer, would require too many stations. When there is 

 an accumulation of a large amount of meteoric material, the magnetic 

 method may be used to locate areas of greatest concentration. For 

 Meteor Crater, Arizona, Jakosky"^ reports anomaUes of surprisingly small 

 magnitude (687) at the SW portion of the crater and assumes that the 

 magnetic material begins at 200 feet and concentrates with depth. 



B. Magnetic Surveys in Oil Exploration 



Oil-bearing formations are rarely magnetic, and some other formation 

 of a known structural or stratigraphic relation is mapped. Such associated 

 formations are (1) salt domes, (2) magnetic beds in the sedimentary sec- 

 tion, and (3) basement rocks and igneous intrusions. The exceptions 

 where the oil-bearing formations themselves produce magnetic effects are 

 (a) granite wash on the flanks of ridges, (6) serpentine plugs and laccoliths, 

 and (c) shoe string sands. 



»" Reich, Zeit. Geophys., 2(7), 273-278 (1927). 



"» Razvedka Nedr., 5/6, 18-21 (1933). 



«» Ibid., 19, 19-21 (Dec, 1933). 



"^ Chromite Deposits of Newfoundland, Dept. of Nat. Res., St. Johns, Nf. 



»«»Min. and Met., 16, 337 (Jan., 1935). 



"« N. Trubiatchinski, Geol. and Prosp. Service, U.S.S.R. Fasc. 166, (1932). 



»^ A.I.M.E. Geophysical Prospecting, 69 (1932). 



