High-pressure compaction studies (up to 386,000 psi) were conducted on halloysite and dickite clays. The remaining moisture content in per cent (dry weight) was plotted versus the logarithm of pressure in psi. For halloysite clay there is a straight-line relationship between 1000 and 260,000 psi (M = 40.1 − 6.66.1ogP), and for dickite clay the relationship between the pressure (psi) and the moisture content (per cent) can be expressed by the formula M = 26.7 − 5.04.1ogP between 200 and 120,000 psi. X-ray analyses were made on compacted samples in order to determine any possible mineralogic changes with increasing overburden pressure. In addition, the permeability of dickite clay at an overburden pressure of 15,000 psi was calculated.