Prof. Sollas—Varieties of Igneous Rocks. ool 
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 perlitic in character. Thus a structure which was sup- 
posed to be confined to glasses that have cooled rather rapidly is 
shown to occur rarely, but occasionally, in crystals. 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 
are for the most part independent. 
1V.—On tHe Minure Srructure oF THE SKELETON oF “ Mono- 
Graptus Priopon.” By Professor W. J. Sotuas, D.Sc., F.R.S. 
[Communicated by permission of the Director-General of the 
Geological Survey. | 
EMAINS of Jonograptus priodon in an exceptionally perfect 
state of preservation occur in the Silurian limestone of Barn- 
ham Hill, Co. Tipperary, and are exhibited 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 une 
author describes the structure of the wall. 
Most of the sections are transverse and display the cen Searerl 
canal and one hydrotheca ; they measure a little over 1:5 mm. along 
the greater, and about 1 mm. along the shorter axis. The wall, 
0-025 mm. in thickness, consists of black carbonaceous material in 
a more or less fragmentary condition, but sufficiently continuous to 
enable the existence of three layers to be determined: an outer and 
inner, which are very thin, separated by a space, now filled with 
calcite, from a thicker middle layer, which measures from 0-005 to 
0-001 mm. across. The middle layer sometimes breaks up into 
threads, and the superficial films have a reticular appearance, which 
may, however, be due to post-mortem changes. In the region of the 
virgula and also along the free edges of the theca the wall thickens, 
partly by an enlargement of the space between the layers, and partly 
by a thickening of the middle layer. Thus, in one example the 
total thickness of the wall in the virgular region is 0-075 mm., and 
of the virgula itself, which represents the middle layer, 0-087 mm. ; 
similarly at the margin of the theca the total thickness was found to 
be 0085 mm., the included middle layer measuring 0-045 mm. 
Thin threads of carbonaceous material extend from the middle to 
the superficial layers, and are particularly obvious in the thickened 
regions. The virgula would appear to possess no independent 
existence ; it seems to be merely a thickening of the middle layer. 
V.—On tHe OriciIn or INTERMEDIATE VARIETIES OF IGNEOUS 
Rocks spy Intrusion anD ADMIXTURE, AS OBSERVED AT BaArna- 
VAVE, CaRLINGFORD. By Professor W. J. Souuas, D.Sc., F.R.S. 
HE two principal kinds of Rocks composing the mountain of 
Barnavave are a dark-coloured, almost black, gabbro and a 
light-coloured, almost white, granophyre. ‘This extreme contrast in 
