Vol. 50.] OF PERLITIC CRACKS IN QUARTZ. 375 



EXPLANATION OF PLATE XVIII. 



All the specimens are from Sandy Braes, 1| mile N.E. of Tardree Moun- 

 tain, Antrim. The numbers refer to the series of sliced rocks in the col- 

 lections of the Geological Survey of Ireland. 



Fig. 1. 1927 a. Polygonal and radial cracks in matrix of pitchstone. Perlite 

 centred in quartz and involving matrix. X 25. 



Fig. 1 a. Outline of quartz in fig. 1. 



Fig. 2. 1927 a. Radial cracks from polygonal and perlitic cracks. Matrix- 

 perl ite, enclosiug two perlites which involve quartz and 

 matrix. X 25. 



25 



Fig. 3. 1 927 a. Polygonal aud spherical cracks in quartz and matrix. X -<> * 



Fig. 4. I 927 a. Spherical cracks in quartz, some of which pass into the matrix. 

 The cracks are filled with a brown deposit, showing reticu- 

 lated texture. X 25. 



Fig. 5. 1927. Matrix-perlite, with outer and polygonal cracks traversing 

 quartz. X 25. 



Fig. 6. 1927. Matrix-perlite, with radial cracks; polygonal and radial cracks 

 traversing both quartz and matrix. X 25. 



Fig. 6 a. Radial cracks at j unction of quartz and matrix. X 125. 



Note. — The drawing of each illustration in this paper has been carefully checked 

 by photographs taken on the same or a larger scale. 



Discussion. 



Mr. Rutley remarked that the careful observations made by the 

 Author appeared to be quite satisfactory up to a certain point, 

 although he should hesitate to describe the cracks in the quartz 

 and olivine-crystals as perlitic. That perlitic structure and the 

 structures seen in these crystals were both due to shrinkage there 

 was no question. Spheroidal structure in basalts and other crys- 

 talline rocks also resulted from the same cause, yet the spheroidal 

 structure of a basalt and the perlitic structure of an obsidian were 

 sufficiently different to deserve different names. He had occasionally 

 met with cracks in olivine-crystals similar to those exhibited, but he 

 had never regarded them as perlitic. On the whole, he considered 

 that the Author had given the Society an extremely interesting and 

 valuable paper, but he did not agree with him in thinking that the 

 facts now brought forward were sufficient to invalidate conclusions 

 hitherto formed with regard to the once vitreous character of rocks 

 showing perlitic structure. In the absence of this structure it was 

 not possible to say whether a felsite originally solidified as a lithoidal 

 or as a vitreous rock, because, if originally vitreous, it might assume 

 a lithoidal character through devitrification. Furthermore, if 

 perlitic structure occurred in a lithoidal rhyolite, there was no means 

 of proving that the rock was lithoidal when the perlitic structure 

 was developed. 



Mr. Harker thought the Author had proved that true perlitic 

 cracks may be formed in a mineral like quartz, possessing no marked 

 cleavage. He compared the phenomena with those recorded in the 



