102 WP. SEN NY 
anticlines, or they may be due to more local stresses and lateral pres- 
sures, as a consequence of the settling of the basin. Probably both 
factors should be considered and some unconformity between deeper 
and shallower structure may reasonably be expected, mainly as the 
result of salt flowage in the Salina. 
Since the glacial drift and the shallow formations do not seem ap- 
preciably to influence the magnetic field, it appears that the magnetic 
anomalies may be mainly attributed either to the structures or to the 
MaGNE TIC ANOMALY AT GRAND RaPiDS, MICH. 
ay W.PJenny 
LOWELL 
CALEDONIA 
E=—S) POSITIVE TREND 
cD CENTRAL AREA OF ANOMALY 
SCALE OF LOCAL MAGNETIC VECTORS 
0 4 8B (200 CAMMA 
SCALE OF MILES 
iN 
ALLEGAN 
t 

FIG. 3 
mineral content of the pre-Cambrian. From a comparison of the vec- 
tor map (Fig. 2) with known structures, we conclude that the ma- 
jority of magnetic anomalies may safely be interpreted in terms of 
structures: positive vectors indicating anticlinal structures, negative 
vectors indicating synclinal structures. 
The negative magnetic vectors at Midland, Mount Pleasant, Big 
Rapids, and Newaygo are indicative of the large depression in the 
heart of the basin. 
The horizontal vector at Ann Arbor points toward a “high” near 
Howell as indicated by the two vectors. The negative vectors at Flint, 
624 
