212 Dublin Microscopical Club. 



the mass of the ashes consisted, displayed under the microscope 

 delicate threads like the well-known " Pole's Hair," and there could 

 be recognized marked crystals of a triclinic felspar, a monoclinic 

 crystalline substance, augitic j)yroxyene, also a rhombic mineral, 

 probably a hypersthene. These minerals, so far as Dr. Frazer knew, 

 were quite distinctive of this " Sand," for he had not observed anj' 

 similar combination in any pumice which he had examined. !Mr. Joly 

 had also investigated this dust and had given a full communication 

 on the subject to the Royal Dublin Society, illustrated with photo- 

 graphs. He found small crystals of iron pyrites and of a mineral, 

 probably bornite ; these were not noticed by Dr. Frazer. From an 

 attentive consideration of the microscopical appearances Dr. Frazer 

 was disposed to conjecture that steam alone was not the eruptive 

 agent ; but probably at a high temperature the steam was resolved 

 into its gaseous elements, thus accounting for the violence of the 

 explosion which took place and for the quantities of miuute porosities 

 visible in the pumice, which in parts recalled to mind the appear- 

 ance of viscous ice, whence particles of imbedded air are gradually 

 escaping. 



March 20, 1884. 



Section of Diorite from Loch Assynt. — Prof. Hull exhibited a 

 section of a peculiar sheet of diorite of intrusive origin found in the 

 limestone of Loch Assynt in the form, of a sheet or dyke. Under 

 a low magnifying-power it is seen to be a beautifully crystalline rock 

 consisting of crystals of hornblende, triclinic felspar, and magnetite 

 imbedded in a glassy paste. The polarization of the minerals was 

 vivid, and in the case of the pyroxenic mineral indicative of horn- 

 blende rather than of augite. 



Structure of Leaves of Selaginella densa. — Dr. M'Nab exhibited 

 preparations of an undetermined species of Selaginella which was 

 cultivated by Mr. Sim of Foot's Cray, Kent, as Selaginella densa. 

 On examination it was obseiTed that stomata were developed along 

 the margins of the leaves as well as in the usual position near the 

 mid-rib. A similar arrangement of marginal stomata occurs in 

 cultivated specimens of Selaginella Poulteri. 



Section of a clastic Hock from Bray Read exhibited. — Prof. V. 

 Ball exhibited a section of a dense purple-coloured rock which is 

 found near the southern extremity of the section of Cambrian rocks 

 forming Uraj' Head. The mode of occurrence of this rock being for 

 the most part obscure, although at one point it is distinctly stratified, 

 this, together with its density and hardness, made it desirable to 

 examine its microscoi^cal characters. It proves to be a distinctly 

 clastic rock, consisting mainly of small fragments of quartz in a 

 ferruginous matrix. It may be regarded as a somewhat exceptional 

 variety of the group of rocks of this age to which the term " grit " 

 used to bo applied by Prof. Jukes. 



