Page:Seven Years in South Africa v1.djvu/216

 further down the valley. Where, however, several fissures are concentrated at one point, they result in the formation of the craters to which I refer. At the top, these funnels are sometimes as much as 200 or 300 yards in circumference: at first sight they have the appearance of being circular, but



on investigation they nearly always prove triangular, or, less frequently, quadrilateral. The interstices of broken rock with which their inner surface is lined are filled up by the surrounding