MAGNETIC METHODS 211 



Illinois, is positive, and a basin that does not have strongly magnetic beds, 

 such as West Texas, is negative. Uplifts that have a thick section of 

 Ordovician limestones, such as the Ozarks, are negative because the flanks 

 have magnetic shales and sands above the limestones. The Nemaha ridge 

 is positive because the granite is sufficiently close to the surface to out- 

 weigh stratigraphic influences. X 



Local anomalies, as the name implies, extend over relatively small 

 areas. The magnetic intensity in these areas may occasionally reach a high 

 value; for example, near Juneau, Alaska, a local magnetic pole produces 

 an anomaly of sufficient magnitude to cause a dip needle to stand vertical. 



Classification of Anomalies. — The geophysicist H. Haalck has given 

 a very convenient classification of magnetic anomalies, dividing them into 

 four classes. 



Class 1 anomalies have a range of from 10,000 to 200,000 gammas of vertical mag- 

 netic intensity. Examples are rare, but probably the most famous one is the extremely 

 large anomaly at Kursk, in Russia, already mentioned. An American example is the 

 extensive magnetite deposit in Boulder County, Colorado.f 



In such magnetic features the magnetic vertical intensity may be several times as 

 great as the value of that component of the earth's magnetic field for the latitude of the 

 locality. Such exceptionally great anomalies arise from large deposits of magnetite, 

 often of economic importance. 



Class 2 anomalies have values of from 1,000 to 10,000 gammas. They are generally 

 due to extensive masses of volcanic or of crystalline rock rich in magnetite. The mag- 

 netic anomaly resulting from the Ralston Dike (basalt) in Jefferson County, Colorado 

 is in this class§. 



Class 3 anomalies are of from about 100 to 1000 gammas. They may be caused by 

 masses of rock of the same kind that gives rise to Class 2 magnetic features, but carry- 

 ing less magnetite. Also, they could be caused by less extensive bodies of magnetite- 

 rich rock. Examples are numerous in the mining districts of the United States, Canada 

 and Australia, from magnetic dikes and ore bodies, and the magnetite concentrations 

 of certain placer deposits. 



Class 4 anomalies include those up to 200 gammas. Haalck considers that these 

 arise from sedimentary formations. The maximum figure should be perhaps 300 

 gammas. 



The magnitude of a magnetic anomaly is an important factor in arriving 

 at a conception of its cause. Another valuable guide in interpretation is 

 the general geology of an area and the type of geologic structure that might 

 reasonably be expected to occur in it. To use a somevi^hat exaggerated 

 example, it v^ould not be geologically sound to interpret a 20,000-gamma 

 magnetic high, if found in eastern Colorado, as representing an anticlinal 

 structure favorable for the accumulation of oil and gas. The magnetic 

 anomaly associated w^ith the large Ft. Morgan anticline in that area is only 

 600 gammas. 



t W. p. Jenny, loc. cit. 



t Heiland, Henderson & Malkovsky; U. S. Bureau of Mines, Tech. Paper 439, pp. 41-49, 1929. 

 Covers geophysical surveys at Caribou, Colorado. 



§ W. S. Levings, "A magnetic survey of the Ralston Dike, Jefferson County, Colo.," Quarterly, 

 Colo. School of Mines, Vol. 27, No. 3, 1932. 



