Lithology interpretation using neutron/density cross-plots requires pure matrix response lines for sand- stone, limestone, and dolomite. In the cases of limestone and sandstone, the neutron response deter- mined from laboratory measurements has been substantiated by field experience. However, in the case of dolomite the interpretation line for the thermal neutron porosity device is based on field experience which is somewhat at odds with laboratory measurements. This so-called "anomalous" response in dolomite has been the subject of earlier studies which proposed trace element thermal neutron absorbers such as boron or gadolinium as a possible explanation. A more plausible explanation is the presence of another strong, yet common, absorber such as chlorine, which is found in nearly all borehole and formation waters.