A PEELITIC PITCHSTONE FROM TWEED EIVEK. did 



in a glassy basalt containing phenocrysts of olivine. Certain 

 curved cracks surround the grains of olivine, but are elsewhere 

 absent. They lack the even curvature of perlitic cracks, but, 

 waiving this point, it is evident that they depend on the olivines 

 for their existence, and do not prove that ( the magma would per se 

 have yielded perlites. It appears to me that this case is on a par 

 with the lithoidal variety of rock referred to by Mr. Watts (Op. cit. 

 p. 373). These cracks, said by the author to be perlitic, surround 

 the quartz grains. But, as in the former case, I should consider 

 these cracks as doubtfully perlitic, and due to the contraction of 

 the quartz, I am still unconvinced that an originally lithoidal 

 groundmass has been found exhibiting of itself perlitic structure. 

 As Mr. Watts points out, quartz possesses a peculiar advantage 

 towards yielding perlitic structure in having no cleavage, and, if 

 such a medium cannot produce anything better than has been 

 brought forward up to the present, it appears to me extremely 

 improbable that a crystalline aggregate, consisting of individuals 

 with various cleavages and different coefficients of contraction, 

 would be able to develop so delicate a structure. 



Reference to Plates. 



PLATE XLIV. 



General view of the rock x 15 diameters. Showing several grains of 

 quartz with curved cracks, a large sanidine twin, and some small grains 

 of magnetite and hypersthene. 



PLATE XLV. 

 Fig. 1 x 30 diameters. A curved crack in the quartz on the right 

 passes out into the glass through a crystal of felspar and round a grain 

 of magnetite. This encloses a perlite, the outer curve of which is visible 

 passing through the felspar. 



Eig. 2. x 500 diameters. Showing the straight felspar microliths and 

 the curved ' granular axiolites.' 



Fig. 3. x 10 diameters. The perlitic structure of the rock photographed 

 from a polished surface. 



Fig. 4. Imitation of the structure of the rock in Canada balsam. The 

 central patch which represents a piece of quartz shows some incipient 

 perlitic cracks in addition to the curved cracks, but nothing like them is 

 present in the quartz of the rock. 



