The collection of rocks from the Sahara to be discussed was made by Captain D. R. G. Cameron, of the Royal Scots, during a journey across the Sahara from Kano to Algeria. The journey is described by Captain Cameron in his book A Saharan Venture (1928) and in a paper published in the Geographical Journal (lxxi, 1928, 538–59). The specimens were not collected in situ, which reduces their value somewhat. When one considers, however, that the collecting area is desert country where outcrops are comparatively scarce it is apparent that conclusions based on pebbles are not without value.