Previous Mars Global Surveyor Thermal Emission Spectrometer (TES) results have identified two principal surface compositions on Mars; basaltic surfaces are found primarily in the southern highlands, while andesitic surfaces are found in the northern lowlands and in smaller concentrations distributed throughout the southern highlands. In 1 pixel-per-degree composition maps, the exact boundary between the basaltic and andesitic compositions is obscured by large amounts of equatorial surface dust. In this work, high-resolution TES surface spectral data are used to identify and characterize isolated regions of basalt within the equatorial dust regions and northern lowlands (generally between 20¿S--60¿N, 0--360¿W) in an effort to better understand the nature of the boundary between the two compositions. The purpose of refining this boundary is to more accurately determine the relationship of the compositional dichotomy with global surface morphology and derived crustal thickness. Results show that the basaltic surface composition is closely associated with the ancient cratered terrain morphology but is not well correlated with crustal thickness. The northern extent of the TES basalt composition is also established. With four exceptions, there are no significant concentrations (>~30% surface cover) of basalt greater than ~400 km2 in area within the northern plains. Three are located within or adjacent to knobby terrain, which has been suggested to be erosional remnants of the cratered highlands. The fourth exception is in Milankovic crater and may represent a local basalt flow or basaltic material excavated during crater formation. Results from this work suggest that there is a stratigraphic relationship between the basaltic and andesitic surface compositions, with younger andesitic materials overlying older basaltic materials throughout the equatorial and northern plains regions.