GEOLOGICAL SOCIETY OE DUBLIN. 
57 
a represents the place of a cliff situate in an island not in the line 
of section, opposite which the rock in the bed of the river exhibits 
joints anticlinal to No. 7. The cliff is formed of a light-coloured syenite 
with whitish felspar and green hornblende; and b shows the position of 
a cliff on the side of the fall, similar in mineralogical character to JNTos. 
5 and 6. 
The phenomena I have tried to describe seem to suggest that the 
syenite here was an altered sandstone, elevated at an axis marked by 
the meeting of the anticlinal planes of the joints; and I consider the 
rocks at this junction no less worthy of observation on that account, 
than for the distinctness with which they show the tendency of a rock 
mass to assume definite crystalline forms under favourable conditions, 
which in this case might be attributed to the absence of pressure on 
the sandstone that had previously been stripped by denudation. 
Mr. Kelly made some remarks on this paper, stating that a rock 
protruded through limestone rendered it dolomitic. 
Professor Galbraith dissented from field geologists characterizing 
dolomitic rocks by colour or crystallization only, as analysis leads us to 
believe that these characters often lead us astray. 
Professor Jukes observed, that the term which should properly be 
used in speaking of these rocks was not dolomite, but magnesian lime¬ 
stone, as he could instance cases in the experience of Mr. Wyley, in 
which the presence of magnesia in as small a per-centage as 5 per cent, 
had been detected by mere inspection alone. 
Professor Harkness considered we must believe that dolomitization 
did not arise from igneous agency, but from forces acting externally to 
the rock, probably from the action of sea-water at great depth, under 
great pressure, by means of the decomposition of the sulphate of mag¬ 
nesia held in solution. 
The Chairman stated the last opinion had often appeared to him to 
explain dolomitization as it occurred in this country, but not to explain 
the great masses of dolomite as they occur abroad. 
Professor Galbraith objected to the theory, as we ought to find sul¬ 
phate of lime along with the dolomite, if it were true. 
Professor Harkness thought the sulphate of lime, being more soluble, 
ought to be carried away in solution. 
Professor Galbraith dissented from this solution. 
Me. J. Beete Jukes also read the following paper :— 
THE DELFT OF WEST GALWAY AND THE EASTEEN PAETS OF MAYO. 
BY JOHN BIEMINGHAM. 
Among the geological phenomena of Ireland not the least interesting 
are the escars, or gravel hills, which are found in all parts of the island. 
They differ from the drifts of other countries by presenting no fossili- 
ferous testimony of their period ; while the similar lines of gravel hills 
in Northern Europe are proved, by their resting on beds containing 
