J. S. G. Wilson 8f H. B. Mnjf—The Hill of Beath. 59 



A thin slice of a specimen from the mine shows under the micro- 

 scope that the lapilli are irregularly rounded masses of greenish-brown 

 palagonite, charged with cumulites and longulites, and without action 

 on polarized light. Pseudomorphs in calcite after olivine are enclosed 

 in some of the lapilli. The abundant, minute vesicles, which have 

 oval shapes and are not often cut by the margins of the lapilli, are 

 filled by zeolites or sometimes by chlorite. la some parts, however, 

 the brown palagonitic substance shows faint aggregate polarization 

 under crossed nicols, whilst the walls of the vesicles and of cracks 

 show a fibrous structure, the very short fibres being arranged perpen- 

 dicular to the walls. ^ This alteration is probably a secondary 

 devitrification, and the vesicles in these altered parts are frequently 

 occupied by calcite. In the dark matrix chips of quartz and felspar 

 and minute particles of palagonite may be discerned, but the structure 

 is in most parts quite obscure. 



The numerous sand-grains in the tuff consist of angular and rounded 

 particles of quartz, with some of microcline and plagioclase. These 

 minerals are the chief constituents of the surrounding Carboniferous 

 sandstones, and the grains in the tuff may have been derived from 

 them, the concave curves bounding many of the angular quartz-grains 

 being readily explained as the trace of fresh conchoidal fractures 

 produced during the volcanic explosions which blew out the strata. 

 But if the grains were really derived from the disruption of the 

 Carboniferous sandstones, the absence of blocks of sandstone and shale 

 is remarkable, unless it may be assumed that the rocks were not 

 thoroughly consolidated at the period of vulcanicity. We refer later 

 to the probability that the neck belongs to the age of the Upper Lime- 

 stones, and supposing the volcano to have been a subaqueous one, the 

 sand-grains may have been derived from the same source as the other 

 Carboniferous sediments and deposited in the neck along with the 

 truly volcanic material. 



The lenticular mass of basic I'ock, which crops out on the eastern 

 side of the neck, was not met with in the mine. The rock is 

 decomposed at the outcrop, and its junction with the tutf is not 

 well exposed, but probably it is intrusive into the tuff. It is a very 

 fine-grained black rock, showing to the eye small, scattered 

 phenocrysts of idiomorphic olivine altered to serpentine. Under 

 the microscope an abundant second generation of idiomorphic 

 olivine phenocrysts rejilaced by serpentine, reddish-brown strongly 

 pleochroic biotite hexagonal in cross-section, and two or three small 

 phenocrysts of pale green augite lie in a groundmass, which is charged 

 almost to opacity by dusty magnetite, and is further obscured by spots 

 of secondary calcite. In the thinner parts of the slide, however, one 

 can see that the groundmass consists of minute prisms of augite, 

 apparently pale green in colour, embedded in a clear, colourless, 

 isotropic substance, which has a very low refractive index and is 

 probably analcime. Recognizable analcime occurs in certain clear areas 

 to be referred to below, and from these areas it appears to be 



^ Zirkel noted a somewhat similar alteration in palagonite tufP fi'om Nevada. 

 Micro. Fetro. Fortieth Farallel, 1876, p. 274. 



