those presenting weaker ($\simlt$ 10%) and more diffuse absorption whose nature is also more uncertain. Figure \[absorption\] presents the ISOCAM images of 11 fields with strong absorption features, to which we added the widely studied core L1544 (e.g. Ward-Thompson et al. 1994, Tafalla et al. 1998, Williams et al. 1999, Ohashi et al. 1999), and from which we withdrew L1709C (the absorption features lying in the corners of the image, making the analysis difficult). The remaining fields are presented in Fig. \[weak\]. Figure \[cut\] shows examples of cuts through two of the deep absorption features (for L328 and L1689B). The morphologies encountered in the fields with compact absorption vary from spherical cores like L328 to more elongated, filamentary structures (e.g. L1689B, GF5).

Comparison with the millimeter continuum maps
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All of the cores observed at 1.3 mm (Table \[mm\]) present extended, weak dust continuum emission which we were able to map. For all cores, the peak of the mid-IR absorption does correspond with a peak in the millimeter emission. This is illustrated in Fig. \[isomm\] which shows the mid-IR ISOCAM absorption contours superimposed on the 1.3 mm continuum maps of the eight cores positively detected at 1.3 mm (the maps 