234 A NATURALIST IN THE PACIFIC CH. xvi 



The vents, from which the materials forming this promontory 

 were ejected, were arranged in a single straight line for a length of 

 14 miles. Along this linear fissure, which was probably submarine, 

 vents were at different times formed ; and though owing to sub- 

 aerial and marine degradation the present surface has been since 

 shaped and reshaped, their situation may still be recognised in the 

 " necks " of tuff and agglomerate that form the peaks, and in 

 the large dykes or sills of quartz-porphyry and oligoclase-trachyte. 



