542 aS*. Allport — On the Igneous JRocJcs of Arran. 



crystals contain many cavities inclosing portions of tlie base, and 

 are invariably surrounded by a band of green spherolites so closely 

 packed together as to interfere with their full development. The 

 felspar is much altered, but a few crystals still display coloured 

 bands and striae. 



A spherolitic felsite appears to form an intrusive vein on the 

 south slope of Dun Fion above the Corriegills shore. It is a 

 rather hard yellowish-grey rook, full of small spherical grains, 

 without imbedded crystals. Under the microscope, with ordinary 

 light, the felsitic base appears to be full of brown dust, with 

 many imperfect belonites scattered through it ; there are also many 

 clear circular spaces, which exhibit no structure whatever. Be- 

 tween crossed Nicols, however, a spherolitic structure is at once 

 evident; the clear spaces then display one or more circular disks 

 traversed by a black cross. This kind of latent, or crypto-spherolitic 

 texture, is the same as that previously described in the pitchstones. 

 There are many small spherolites of distinctly fibrous structure. 



We have previously seen that the brown pitchstone from King's 

 Cove passes into a felsite, and in the rocks just described we have 

 examples of typical felsites containing spherolites and belonites, 

 differing in no respect from those common in pitchstones. 



On the Lamlash road, abovit a mile from the Brodick Hotel, there 

 is a mass of spherolitic felsite in connexion with the vein of pitch- 

 stone, which is well exposed in the bed of the stream below. It is 

 the bed of " claystone " mentioned by Dr. Bryce, who describes the 

 relative positions of the two rocks, and observes, that " the relations 

 of the two beds lend countenance to the idea that these claystones 

 are but altered sandstones." It is by no means easy to understand 

 how any one who had given some attention to igneous rocks could 

 entertain such an idea. In external appearance the rock is not 

 unlike a nearly white Oolitic limestone, full of small yellowish 

 grains. Under the microscope a thin section shows the grains to 

 be spherolites of fibrous radial texture, thickly scattered through a 

 felsitic base containing many distinct grains of quartz. The rock is 

 somewhat decomposed, but a few of the spherolites still exhibit 

 traces of belonites. 



A crystalline felsite from Auchenhew Hill, about a mile and a 

 half N.W. of Kildonan Castle, is an interesting rock, as it exhibits 

 a composition and structure combining the characters of the felsites 

 with those of the more perfectly crystallized dolerites, and may, 

 therefore, be compared with the well-known trachy-dolerites. It is 

 of a light grey colour, distinctly crystalline in texture, and contains 

 a few cr^'stals of orthoclase. The felsitic base contains grains of 

 quartz, and many spherolites, like the preceding rocks ; but there 

 are in addition numerous small felspar crystals, and long prisms of 

 a brown pyroxenic mineral, too much altered for determination : it is 

 probably altered hornblende. There are also many black grains of 

 magnetite ; the larger felspar crystals are twins of orthoclase. This 

 rock occurs high up among the sandstones on the south side of the 

 hill ; the latter being capped by a thick sheet of columnar dolerite. 



