W. W. Watts— Perlitic Structure. 17 



As perlitic structure is by no means of constant occurrence, even 

 in glassy magmas, the author seeks the reason from analogy with 

 balsam and finds it in the friction due to the cooled surface ; 

 " doubtless these only occur where there are some particular il- 

 lations between the strain and the viscosity, or brittleness, of the 

 medium." 



Turning now to the ' perlitic cracks ' in the quartz, these differ- 

 ences are noticeable: "First, they do not lie in the meshes of a set 

 of polygonal cracks, but on the contrary are often seen to be con- 

 tinuous with some of the polygonal cracks of the matrix. Second, 

 instead of being segmental or touching each other tangentially they 

 are almost invariably closed by abutting abruptly against each 

 other." On the other hand, "perhaps the most obvious characteristic 

 is that the curves constituting a (matrix) perlite never meet other- 

 wise than tangentially, while another striking feature is the fact that 

 the curves are so frequently segmental, the crack ending abruptly 

 in the medium, and even curving round so as to run parallel with 

 an adjacent crack instead of splitting into it." Mr. Smeeth therefore 

 endeavours to imitate quartz cracks and succeeds very satisfactorily 

 by cementing on the ground glass round slips of cover-glass and 

 pouring balsam on both the smooth and rough glass. Then, on 

 heating and cooling in the usual way, it is found that the cracks 

 over the smooth glass are quite distinct from those over the ground 

 glass, and the former present a very strong likeness to those seen in 

 quartz. Anyone who compares the figure of this structure given by 

 Mr. Smeeth on plate xlv of his paper with figure 2, p. 371, and 

 figure 1, plate xviii of mine, 1 will be struck with the obvious points of 

 resemblance between the cracks in quartz and those in the balsam. 

 Unfortunately the balsam figure is not quite sufficiently distinct to 

 institute a very minute comparison, and we cannot be sure that the 

 cracks pass outward from the balsam over the smooth to that over 

 the rough glass ; apparently they do not. Again, none of the 

 characteristic ground-glass cracks penetrate into the rest. These 

 features are both very marked in the rhyolite of Tardree. I am, 

 however, truly glad that Mr. Smeeth has shown that there is strong 

 ground for believing that both types of cracks are due to shrinkage 

 on cooling, and that they are the result of a single continuous pro- 

 cess acting on the same type of material under slightly altered 

 circumstances, though he does not seem to have followed this idea 

 to its logical issue by indicating that these aberrant cracks will 

 indubitably be produced at times in pure glass, and that such cracks 

 in glass have often been referred to as perlitic. In conclusion the 

 author points out that certain of the cracks described by me as 

 occurring round, and often cutting, quartz grains are probably the 

 result of shrinkage of the quartz alone, and, while cracks are likely 

 to be produced in a homogeneous substance like quartz, he regards 

 it as very unlikely that anything even so far approaching perlitic 

 structure could ever be produced in a heterogeneous lithoidal 

 ground-mass. 



1 Op. cil., pp. 367-76. 



DECADE IV. VOL. III. XO. I. 2 



