i860.] J. GEIKIE — CARRICK, AYRSHIRE. ,519 



this only, that it never is pebbly or granular. They vary in diameter 

 from a few inches to a foot and upwards across. In the smaller 

 areas the form assumed is irregular ; but as they increase in size they 

 become bomb-shaped or rudely spherical. The amygdaloidal vesicles 

 are always round, save where two or more have coalesced to form 

 a confluent ragged cavity. They vary in size from that of the finest 

 seed up to vesicles with kernels like small sweet, peas. Larger 

 cavities than this are rare. The '' sweet-pea " cavities are for the 

 most part aggregated towards the centre of a vesicular area, the 

 vesicles decreasing in size as they approach its circumference. In 

 this way each little amj^gdaloidal area shades off imperceptibly into 

 the unaltered greywacke in which it appears to be imbedded. Outside 

 of such an area the greywacke has usually a pasty aspect, not unlike 

 the dull matrix of some highly basic trap, but not infrequently a 

 granular texture ; and even small specks of dark shale may be 

 obtained only a few inches from the amygdaloidal portions of the 

 rock. The vesicular areas most commonly occur not far from each 

 other, but often they lie widely apart. We may obtain them in 

 contact, or from an inch to several yards asunder, the intervening 

 parts of the bed being always destitute of amygdaloidal structure, 

 and frequently showing a granular and sometimes even pebbly tex- 

 ture. 



The rocks which have just been described exhibit well-marked 

 bedding, and alternate in places with coarse shales. The softer 

 beds weather away, leaving the harder bands isolated; the joints 

 are like those of typical greywacke, square, sharp, and well defined. 

 In short, the general appearance of the beds is quite that of the 

 Lower Silurian rocks in the unaltered regions of the surrounding- 

 country. 



3. AytiorpTious Green Paste-rock. — In the peculiar masses that 

 have next to be considered all trace of bedding has disappeared ; the 

 jointing is obscure and ramifying, and the general aspect of the 

 rocks as unlike any regularly stratified deposit as it could well be. 

 The masses referred to consist of a greenish pasty matrix, thickly set 

 with felspathic bombs. At first sight the rock closely resembles a 

 volcanic ash, and, as seen from the cliffs above, one would scarcely 

 hesitate to call it so. The tough felspathic matrix weathers away 

 with a rough, almost trachytic surface, leaving the bomb-like stones 

 standing out in relief. It most usually assumes a dirty greenish hue, 

 sometimes a dark brown. The texture is dull, like that of those por- 

 tions of greywacke already described as encircling a vesicular area. 

 "We may define the matrix shortly as a greenish alkaline felspathic 

 paste in which no distinct crystals are apparent. The pseudo- 

 bombs are of all sizes, from fragments only an inch or two across to 

 blocks several feet in diameter. When broken, their weathered 

 crust is found to be only a line or so in thickness. They consist of 

 purplish-blue and brownish-red felstone, fine-grained, and look as 

 if they might all have been derived from one bed of felstone. The 

 usual globular amygdaloidal cavities are also characteristic of nearly 

 all the pseudo-bombs. These pseudo-bombs are therefore considered 



VOL. XXII. — PART I. 2 N 



