Shrewsbury. — On the Auckland Volcanoes. 369 



busch, and others in explanation of the oHvine nodules occur- 

 ring in the Tertiary basalts of Europe already referred to, 

 rather than by the " inclusion hypothesis " held by Bischof 

 and Daubree with respect to the same. 



2. Scoria Cones and Craters. — Passing on to the second 

 class of these volcanoes — the scoria-cones — we find they are 

 more numerous than the visible tuff-cones. They are steeper 

 than the tufi-cones, but only moderately steep, the angle of 

 their sides varying from 30° to 40°. Now, in many of the 

 hills which have been cut into for the supplies of rough metal 

 for the roads, the back or sides of the cuttings have been 

 sloped down in the course of removal of the material, so that 

 the scoriae are resting at their natural angle of repose. This 

 angle is found to be about 36°. In other words, the slope of 

 the hills as we see them is approximately the same as the 

 slope of loose scoriae which have assumed a position of rest. 

 This proves that the hills have not been disturbed to any 

 great extent by either elevation or subsidence since the 

 materials composing them first settled down after eruption. 



The walls of several of the cuttings show a rude semblance 

 of bedding in their materials, which is rendered more apparent 

 where layers of different degrees of coarseness alternate with 

 one another. This stratification or bedded structure affords 

 a further illustration of the words of Judd {vide "Vol- 

 canoes," chapter v.) : indeed, these cones form a very perfect 

 parallel in nature to the artificial cone described by him. 

 The layers or beds of volcanic material slope at an angle 

 which gradually increases from the bottom to the top of the 

 hill. Of course, a single cutting is usually too small a section 

 of the beds to show this clearly ; but this we do find : that in 

 cuttings near the bottom of any of the hills the beds dip at 

 lower angles than in cuttings at a relatively higher position — 

 for example, near the foot of Purchas Hill the dip of the 

 beds is about 20°, whereas about half-way up Mount Welling- 

 ton the dip is over 30°. The reason of this is that the volcanic 

 cone is formed by repeated and successive additions of ashes 

 and lava to its surface, the material ejected falling thicker 

 and faster towards the centre of eruption. As the cone grows, 

 therefore, the ashes and lava are laid down upon progressively- 

 steeper slopes. This structure accords with the remarks of 

 Professor Geikie and other geologists on the growth of 

 volcanoes ; but there is this discrepancy : In sections illus- 

 trating this formation (for example, Geikie' s diagram section 

 of a normal volcano"'') the material is usually represented as 

 in fig. 1 below — that is, in beds sloping at a constant angle. 

 Such a structure as this would imply that the eruptive force 



* " Textbook of Geology," 2nd ed., p. 225, fig. 47. 

 24 



