18 



IF. IF. Watts— -Perlitic Structure. 





I must take this early opportunity of stating that the slides from 

 Tardree which I have examined do not exhibit anything like the 

 perfection of perlitic structure seen in the well-known examples 

 from Meissen, from Mexico, or even in the instances described by 

 Mr. Smeeth. and it may at once clear the issue if I plead guilty to 

 not having used the term perlite in the restricted sense favoured by 

 this observer: my use of the term will be made plain shortly. 



Tbe main point that I had before me in writing my paper was 

 this : to show that the fact that the matrices of many litboidal 

 felsites showed a rude perlitic structure was not in itself absolute 

 and sufficient proof that the rocks had necessarily once been glassy in 



Section of Perlitic rock from Jalisco, iu Mexico. 



texture. I hoped that I had proved the following points: (1) That 

 curved cracks as perfect as those found in felsites occur in quartz 

 and olivine. (2) That these and other curved cracks pass from one 

 class of material to another, in this instance from quartz to glass 

 and vice versa, with very slight deviation in direction, (3) That 

 all the types of cracks which make up what I might call the 

 ' perlitic system ' of the particular glass in question were to be 

 recognized in the quartz and in most cases passing from glass 

 to quartz. 



The first point, I take it, would not be disputed by Mr. Smeeth. 

 although he remarks, and quite correctly, that, a careful study would 

 discriminate between the types of curves characteristic of quartz 

 and those spiral ones formed when the structure is most favourably 



