TRANSACTIONS OF SECTION C. 781 



often decomposes to a sort of clay containing nodules of less altered rock, so that 

 it has been supposed that toadstone in some localities * replaces ' a bed of clay in 

 others. For this reason, and also because of the loose way in which the word is 

 used by miners, statements as to the number of beds of toadstone and of the 

 presence or absence of ore in it must be accepted with reserve. Careful mapping- 

 over the whole district will be necessary to ascertain the actual number of beds. 

 Two at least may be seen exposed in several places, and there may be three or even 

 four beds. The Black Hillock shaft has been supposed to be one of the vents 

 through which the toadstone came up to the surface, because the bottom of the 

 rock was not reached. Farey, however, maintains that this bed was sunk through, 

 and a careful examination of the mine heap and shaft shows that the dolerite is 

 not coarse-grained, and that there is no trace of agglomerate or of tuff. An 

 occurrence of lead ore in the toadstone of the Wakebridge mine was next de- 

 scribed. The rock in which the ore occurred when examined under the microscope 

 proved to be a decomposed olivine-dolerite. The ore was as good in the toadstone 

 as in the limestone. That the toadstone is contemporaneous with the limestone i» 

 proved by it being interbedded with the latter, by the occurrence of stratified tuffs 

 in various parts of the district, and by the non-alteration of the beds immediately 

 above the igneous rock, though in one or two places a clay bed below it has been 

 caused to assume a columnar structure. 



Very many specimens have been collected from all the outcrops of toadstone, 

 which are some fifty in number, and many of them have been examined under the 

 microscope. The lavas consist mainly of olivine-dolerite, the augite being both 

 in ophitic plates and in irregularly shaped grains. The rock is much more fresh 

 and less amygdaloidal than has been generally supposed. The tuffs are in some 

 cases well preserved, and the outlines of the lapilli very clearly defined. The 

 author hopes shortly to finish the examination of these rocks, and to offer the 

 details to the Geological Society. 



3. Note on the Perlific Quartz Grains in RhyoUte. 

 By W. W. Watts, M.A., F.G.S. 



[Communicated by permission of the Director-General of the Geological Survey.] 



The author exhibited specimens of that variety of the Sandy Braes Rhyolite 

 from County Antrim which was formerly called Perlite. A microscopical exami- 

 nation of the rock shows crystals of sanidine and grains of quartz embedded in a 

 brown glass. The latter shows perlitic structure in great perfection. In addition, 

 however, the grains of quartz exhibit a series of cracks, which are distinctly per- 

 litic in character. Thus a structure which was supposed to be confined to glasses 

 that have cooled rather rapidly is shown to occur rarely, but occasionally, in crys- 

 tals. Hitherto only one case has been observed in which the cracks entered from 

 the crystals to the matrix ; the perlitic cracks in the two constituents for the most 

 part are independent. 



4. On the Minute Structure of the Skeleton of ' Monograptus Priodon.* 

 By Professor W. J. Sollas, B.Sc, F.B.S. 



[Communicated by permission of the Director-General of the Geological Survey.] 



Remains of Monograptus priodon in an exceptionally perfect state of preserva- 

 tion occur in the Silurian limestone of Barnham Hill, Co. Tipperary, and are ex- 

 hibited in the official collection of the Geological Survey in Dublin. These have 

 been examined in thin slices under the microscope, and as a preliminary result the 

 author describes the structure of the wall. 



Most of the sections are transverse and display the ccenosarcal canal and one 

 hydrotheca ; they measure a little over To mm. along the greater, and about 

 1 mm. along the shorter axis. The wall, 0-026 mm. in thickness, consists of black 



