ASSOCIATED IGNEOUS ROCKS OP THE AYRSHIRE COAST. 781 



Towards the base the rock has a rather slaggy aspect, and is slightly 

 vesicular. It generally exhibits an irregular jointed structure, the 

 most developed planes being rudely parallel to the base ; is occa- 

 sionally slightly columnar and often rudely spheroidal, this structure 

 being in one part very marked. Sometimes, also, curved subsphe- 

 roidal joints may be seen. The spheroids are often traversed by 

 radial cracks. "Where parts of the shells have fallen away, and 

 fragments have been left projecting from the rotten, dusty, or 

 pasty material which has accumulated from decomposition, the rock 

 has the look of a breccia. This, however, in the ordinary sense of 

 of the word, I am convinced it is not. By clambering along the 

 screes towards the south we found the rock become decidedly por- 

 phyritic, the crystals of the plagioclase felspar often being about 

 -J- inch long. Good specimens of the rock were difficult to obtain, 

 as it was very tough and traversed by minute concealed joints. 

 Erom the evidence in the field alone I should not hesitate to con- 

 sider this a true igneous rock, and probably a lava-flow. This view 

 is fully confirmed by microscopic examination. The following is 

 the description of a slide cut from the more porphyritic part : — 



The slide exhibits a very minutely granular brown base, almost 

 homogeneous and nearly opaque, probably consisting of a decomposed 

 glassy felspathic matter stained with iron peroxide. This base is 

 crowded with felspar microliths, together with rather irregular spots 

 of viridite ; and in it are scattered some small, but distinct, grains of 

 magnetite and some larger felspar crystals. The felspar microliths 

 are commonly about 0-005 inch, or a little less, in length. The 

 majority are certainly plagioclase, but there are some which may be 

 orthoclase. The viridite is very clear, only now and then containing 

 a little dark dust, and the mineral which it has replaced crystallized 

 subsequently to the felspar. The larger crystals of felspar are a 

 good deal decomposed, exhibiting spots of viridite and other endo- 

 morphs, many probably of secondary formation. The felspar, in 

 many cases at least, has been plagioclase. The rock, then, may be 

 safely named porphyrite. 



The above description will in most respects serve also for the 

 northern rock. I had no doubt when on the ground that it too was 

 a lava- flow ; and the microscopic structure of a porphyritic variety 

 is as follows : — 



The specimen is of a darker and greener colour than the Bal- 

 creuchan rock, and exhibits crystals of whitish plagioclase and some 

 dark irregular spots. Under the microscope the structure is seen to 

 differ in several respects from the above rock. There appears here to 

 have been but little base or cryptocrystalline ground-mass. The slide 

 mainly consists of plagioclase felspar in small crystals, augite, much 

 being still unaltered, and viridite, together with some iron peroxide 

 and a granular brown mineral. In this, as a ground-mass, are scat- 

 tered larger felspar crystals. This mineral is much altered, earthy 

 in aspect ; but enough remains to show that at any rate the majority 

 of the crystals, small and large, are plagioclase. The augite is very 

 little affected, is clear and almost colourless, and shows bright 



