A successful correlation between floor and roof rocks is needed to study the effects of cooling rates on the nucleation, solidification, and trace element content of cumulate igneous rocks. In felsic intrusions, the most obvious assumption to test is that the core compositions of plagioclase feldspar correlate, because the mineral is abundant and resistant to subsolidus exchange. A test of that assumption is provided by the modal abundance of apatite, which peaks near a plagioclase composition of An 38 in both floor and roof rocks of the Kiglapait Intrusion. The test is made adequately sensitive by dividing the measured mode of apatite by an independently estimated residual porosity, a procedure which helps correct for large amounts of trapped liquid that produced apatite in the roof rocks. The procedure is applicable to other intrusions and its success here suggests that failures of correlation found elsewhere deserve another look.