13.2 How do we know ... that mountains have roots?
Fig 13.7
The problem was that the mountains seemed to have too much mass. Pratt modified his hypothesis to include the mountains having a root of anomalously low-mass rock (much like in Fig. 13.6a). This was re-thought by British astronomer George Airy who hypothesized that perhaps it was unusually thick crust (much like in Fig. 13.6b) that was the cause.