A PEELITIC PITCHSTONE FKOM TWEED RIVER. 307 



The Quartz occurs in rounded grains, which however, in many- 

 cases, can be recognized as more or less corroded bi-pyramidal 

 crystals. In parts they are yellowish in colour owing to numer- 

 ous cracks which are filled with a ferruginous staining material. 

 These cracks are generally spheroidal and cause the crystals to 

 break up into small grains with curved faces. 



The Felspar is chiefly sanidine in glassy tabular crystals show- 

 ing the usual transverse cracks and frequently simple twinning. 

 A certain amount of plagioclase (albite) is present in smaller 



grains. 



Microscopical Investigation. 



Thin sections of the rock appear clear and fresh where not 



affected by weathering, and but little secondary alteration is visible 



either in the glass or in the phenocrysts ; the latter, however, 



have undergone more or less corrosion by the molten magma. 



The quartz grains are chiefly remarkable for the numerous 

 strongly curved cracks which traverse them. In sections these 

 cracks are sometimes rudely concentric for portions of their length, 

 but more generally exhibit a number of curves abutting abruptly 

 against one another. Occasionally these cracks include portions 

 of the glassy matrix. Cracks quite similar to these have recently 

 been claimed to be 'perlitic,' and if 'perlitic' simply implies that a 

 crack is curved and may reasonably be inferred to be due to contrac- 

 tion on cooling, then the majority of these cracks are undoubtedly 

 perlitic. I hope to show later on that typical perlitic cracks 

 present well marked characteristic features, which give them the 

 right to be considered as forming a distinct and definite rock- 

 structure, and which features are markedly different to those 

 exhibited by the cracks in the quartz. 



Inclusions are not numerous, and so far as I have observed are 

 all glassy. In sections at right angles to the vertical axis these 

 inclusions are hexagonal in shape with slightly curved sides which 

 may be due to action by the re-fusion of the contained glass. This 

 glass contains minute crystallites, occasionally one or two needle- 

 shaped crystals and invariably a small bubble with a dark border. 



