Typical thermal demagnetizations diagrams of Early-Middle Jurassic sandstones of Irkutsk coal basin. It shows well the presence of normal and reversed polarities (a). Plot (b) shows great circle analysis. Zijderveld plots are in tilt-corrected coordinates; closed/open symbols are projections onto the horizontal/vertical plane; temperature steps are in degrees C. From Kravchinsky et al. (1999).