Stresses acting on an infinitesimal soil element are shown in the figure (with $\sigma_{z} > \sigma_{x}$). The major and minor principal stresses are $\sigma_{1}$ and $\sigma_{3},$ respectively. Considering the compressive stresses as positive, which one of the following expressions correctly represents the angle between the major principal stress plane and the horizontal plane?

1. $\tan^{-1} \left( \frac{\tau_{zx}} {\sigma_{1} – \sigma_{x}} \right)$
2. $\tan^{-1} \left( \frac{\tau_{zx}} {\sigma_{3} – \sigma_{x}} \right)$
3. $\tan^{-1} \left( \frac{\tau_{zx}} {\sigma_{1} + \sigma_{x}} \right)$
4. $\tan^{-1} \left( \frac{\tau_{zx}} {\sigma_{1} + \sigma_{3}} \right)$