74 PEOCEEDINGS OF THE GEOLOGICAL SOCIETY. 



which are generally haematitic pseiidomorphs after some pyroxene, 

 occasionally after hornblende or mica, may often be observed. The 

 usual porphyritic constituents are plagioclase felspars, occasionally 

 in abundant tabular crystals measuring half an inch or more across, 

 also one or more pyroxenes (augite, enstatite), and sometimes brown 

 mica. Where large felspar-crystals occur in a compact green 

 matrix, the rock assumes a resemblance to the verde antique of the 

 ancients. 



The texture of the porphyrites occasionally becomes faintly 

 resinous, where a considerable proportion of glass still remains- 

 undevitrified, as in the well-known varieties from the Cheviot Hills, 

 and in another variety in the Ochil Hills near Bridge of Allan. It 

 sometimes presents a nodular or coarsely perlitic character, weather- 

 ing out in nut-like balls, like the rock of Buckham's Wall Burn in 

 the Cheviot Hills. Much more frequent is a well-developed amyg- 

 daloidal structure, which indeed may be said to be the chief charac- 

 teristic of these rocks as a whole. The steam-vesicles, now filled 

 with agate, quartz, calcite, or zeolite, vary in size from mere granules 

 Tip to large irregular cavities a foot or more in diameter. Where 

 the kernels are coated with pale green-earth and lie in a dark brown 

 matrix they give rise to some of the most beautiful varieties of rock 

 in any volcanic series in this country, as may be seen on the Ayrshire 

 coast at Culzeau and Turnberry. Some rocks contain the vesicles 

 only as rare individuals, others have them so crowded together as to 

 form the greater part of the cubic contents of the mass. When the 

 infiltration-products have weathered out, some of the amygdaloids 

 present a striking resemblance to recent slaggy brown lavas ; lumps 

 of them must have been originally light enough to float in water. 



My colleague in the Geological Survey, Mr. J. S. Grant Wilson, 

 some years ago made for me a large series of determinations of the 

 specific gravity of the volcanic rocks of the Lower Old Red Sandstone 

 of Scotland. He found that the porj)hy rites collected from various 

 districts to illustrate the more typical varieties of rock averaged 

 about 2-66. He also made a series of chemical analyses of a number 

 of the same rocks from the Cheviot Hills, where they are well pre- 

 served. The results are shown in the following Table : — 



