Earlier work has shown that measurements of the anisotropy of magnetic susceptibility (AMS) of Deep-Sea Drilling Project (DSDP) igneous rocks can contribute to resolution of the mode of emplacement with 80% confidence. In order to clarify the reason for this 20% failure rate we have made detailed AMS analyses on traverse through pillow basalts collected during cruises of the R/V Trident. Although extrusive AMS characteristics dominate, occasional pillows possess intrusive AMS features towards the pillow center. This can be explained by magnetic crystal elongation (growth or realignment) occuring in response to a directed thermal stress in the cooling pillow, such as could occur during implosions. We have analyzed 182 DSDP igneous samples and found that the ratio of extrusive to intrusive samples in the entire collection is about 2 to 1. When the limited depth in the oceanic crust from which the samples were taken is considered and when a comparison is made with the measured rate of increase of intrusive fraction with depth in Iceland and the Troodos Massif of Cyprus, the following three possibilities emerge: (1) the intrusive fraction is much higher in oceanic crust than in oceanic islands and the igneous part of ophiolite complexes, (a) a substantial fraction of layer 2 is much thinner in the oceanic crust than in the Troodos Massif, or (3) the DSDP basalts which have been sampled, particularly prior to leg 34, are to some extent not true basement but represent intrusives into layer 1 during later phases of igneous activity outside the midocean ridge. This last possibility is considered to be the most probable explanation of the AMS results.