10 The N.Z. Journal of Science and Technology. [Jan. 
the other hand, took its present course in a different way. The river 
flowed formerly in a well-proportioned valley, graded and widely opened ; 
but into and down this valley there flowed a stream of lava from one of 
the many vents which were active in North Auckland at a not very distant 
period. The river was thus forced to flow over the surface of the solidified 
lava in the bottom of the valley and to cascade over its edge. 
In the case of the Niagara the layer of rock under the limestone 
is a relatively weak shale, which is easily excavated by the splash and 
swirl of the descending water. As the shale is thus removed the lime¬ 
stone in the edge of the fall overhangs, and from time to time blocks 
of it fall away. Thus the edge of the fall being constantly renewed 
is always fresh and sharp, and the fall retreats rapidly up-stream, the 
river below the falls flowing in a narrow, steep-walled gorge, on the sides 
of which may be seen the outcropping edges of the limestone and the 
underlying shale. 
Fig. 2.— Wairua Falls, from below. 
1 In the case of the Wairua the conditions at first sight appear dissimilar. 
Above the falls, it is true, the channel is shallow (see fig. 1), while below 
there is the typical steep-sided gorge (figs. 1 and 3). Clearly, therefore, the 
fall has retreated up-stream. Again, the fall is sharp-edged, even over¬ 
hanging (see figs. 1 and 2). There is, however, no difference in the kind 
of rock forming the edge of the fall and that in the bottom of the gorge 
below : it is all basaltic lava. 
When the rock below the fall is looked at more closely, however, it is 
seen that the deeper part of the lava-sheet is weaker than the upper part; 
for the deeper part is fissured, being divided by the fissures into basaltic 
columns (see fig. 3). The fissures do not extend into the upper part of the 
lava, which is therefore much more resistant to erosion. 
The fissured layer is analogous to the weak shale, and the upper, massive 
lava to the resistant limestone of Niagara; and thus, as the columnar basalt 
is removed by erosion, the layer above, being unsupported at the edge, 
