348 A NATURALIST IN THE PACIFIC chap. 



Note on the changes produced through the hydration 



of palagonite. 



Most of that which is detailed below is not according to my 

 views palagonitisation, but the effect of hydration in the disinte- 

 gration of this material. The initial molecular condition and 

 the other characters which represent potentially the palagonitic 

 change are not connected with hydration ; but are concerned with 

 the causes before explained that led to the formation of a basic 

 glass of such an unstable constitution. Indeed, there is good 

 reason to believe that the changes to be now described may be 

 observed under the ordinary influences of weathering in a wet 

 region. 



The early stages of alteration are well displayed in some of 

 the tuffs formed mainly of basic vacuolar glass, the submarine 

 character of which is often indicated by a few tests of foramini- 

 fera. Whilst the glass retains its original bottle-green colour, it 

 loses the clean sharp conchoidal edges and displays rough and 

 uneven or granular borders. With a high power the surface of 

 the fragment is seen to be minutely pitted or pock-marked in 

 places, the shallow circular pits, less than "Oi mm. in diameter, 

 being sometimes arranged in a row like a number of overlapping 

 rain-prints. This process proceeds until all the surface is affected, 

 and from this cause there is often an appearance of polygonal 

 markings. The pock-marking, however, continues ; and as the 

 pits encroach more and more on each other an irregularly wrinkled 

 rough surface results. Up to this time the glass has retained 

 much of its original colour ; but its clearness is replaced by 

 turbidity, and collections of very minute rounded, rod-shaped, and 

 irregular granules, composed of a colourless feebly polarising 

 material, are displayed here and there in its substance, whilst 

 some of the previously empty vacuoles are now filled with water. 



In the next stage the hydration of the iron-oxides begins, 

 and the glass becomes opaque and yellowish or reddish-brown, 

 and has a more granular appearance, polarising feebly. Cracks 

 now traverse the substance, and penetrate into the vacuoles, which, 

 as they become filled with the alteration products, whether pala- 

 gonitic, zeolitic, or siliceous, become ruptured and curiously dis- 

 torted. The hydration and consequent disintegration continue 

 until the deep stain of the iron-oxide is removed, and a semi- 

 pulverulent whitish material remains. This is the history of the 



