SECT. 1] THE MAGNETIC FIELD OVER THE OCEANS 195 



zones of the north-east Pacific, the Mendocino and the Murray. The former is 

 by far the most spectacular topographic feature in the area ; the bottom rises 

 about 1 km from south to north across it and over most of the map it forms an 

 asymmetrical ridge with a south-facing escarpment 5000-10,000 ft (l|-3 km) 

 high. It marks the boundary between the deep plain of the north-east Pacific, 

 a comparatively smooth abyssal plain 2100-2600 fm (4-5 km) deep, and the 

 shallower ridge and trough province, an area of narrow north- or north-east- 

 trending ridges and troughs, to the north. The Murray Fault, about 6° to the 

 south, divides the deep plain from the slightly shallower Baja California Sea- 

 mount Province. A relatively minor topographic feature, the Pioneer Ridge, 

 crosses the deep plain about 180 km south of the Mendocino Fault. Fig. 12 

 shows the relation of these features to the area covered by the map. Fig. 11. 



The change in elevation across the Mendocino Fault suggests a major change 

 in crustal structure, and this is confirmed by seismic refraction measurements. 

 On the south side of the Mendocino Fault the seismic results are consistent with 

 a normal oceanic crust, consisting of up to a few hundred metres of un- 

 consolidated sediments, a "second layer" 1-2 km thick and generally thought 

 to consist of volcanic rocks and consolidated sediments, and the main, pre- 

 sumably basaltic, crustal layer about 5 km thick. The Mohorovicic discontinuity 

 lies at a depth of about 1 1 km below the surface of the sea. On the north side, 

 where the bottom is shallower, the crust appears to be thinner than it is to the 

 south, which is contrary to what would be expected if opposite sides of the 

 fault are in isostatic equilibrium ; however, since velocities beneath it are 

 rather less than what is considered normal for the mantle, there is some doubt 

 as to whether the Mohorovicic discontinuity has in fact been reached. It may 

 be noted that the line of the Mendocino Fault marks the approximate boundary 

 between various physiographic provinces of the neighbouring continent 

 (Eardley, 1951, p. 430). 



One of the most impressive features of the map is the vastness of the area 

 covered by what is essentially a single, homogeneous, north-south-lineated 

 magnetic pattern. This pattern is known to extend for more than 2500 km in 

 the north-south direction and although its western limit has not yet been 

 determined, it probably extends for a like distance to the west of the coast. 

 It is, therefore, a major feature of the earth's surface. 



At the known faults, and at other places for which there is otherwise no 

 evidence for faulting, the pattern of the anomalies, though generally not their 

 direction, changes abruptly. At the Murray Fault a 200-km section of the 

 pattern on the north side can be matched, with a fair degree of certainty, 

 against a similar section on the south side, in a way suggesting a right-lateral 

 displacement of about 150 km across it. There is no obvious match of the 

 jjattern across either the Pioneer or the Mendocino Faults, within the limits of 

 the map. However, the fortunate fact that the lineations of the field are approxi- 

 mately normal to the faults makes it possible, with comparatively little effort, 

 to extend the map in the vicinity of these faults for the purpose of looking for 

 such a match. This has been done (Vacquier, 1959 ; Vacquier et al., 1961 ; Raff, 



