GEOLOGICAL SOCIETY OF DUBLIN. 
19 
lead you over the same ground which my valued friend, Mr. Griffith, 
formerly tracked for you, well knowing that no one will he more ready 
than himself to hail with pleasure any true additions to our knowledge of 
facts or any well-grounded extension of our theoretical opinions. 
The craggy eminence known as Arklow Eock is about two miles 
south of the town of Arklow.' Its summit rises to a height of 411 feet 
above the sea, from which it is a quarter of a mile distant, and forms 
nearly the southern termination of an elevated rocky tract of ground 
about a mile in length from north to south, and the same distance from 
east to west As soon as the ground sinks on the land side to the level 
of 150 feet above the sea, or thereabouts, the rocks become concealed by 
the Pleistocene deposits which cover all the lower parts of the adjacent 
country. Of these, the well-known “ Marl” is the most conspicuous 
portion. 
The district is divided into two townlands, “ Eock Little” on the west, 
and ‘ ‘ Eock Big’ ’ on the east. ‘ 4 Eock Little’ ’ has a craggy knoll rising to a 
height of about 250 or 300 feet, which is composed of black and dark gray 
slate with some gritstone. In Mr. Wyley’s notes Graptolites were said to 
have been found in this. The strike of these slates is about N. 30 E., while 
that of the cleavage is E. 301ST., the dip of the latter in one place certainly 
was westerly at 70°; but that of the slates was not so easily determined, 
from the smaEness of the exposed portion of rock. Everybody of ex¬ 
perience in slaty countries knows how little dependence can be placed 
on observations of bedding made on surfaces of a less depth than twenty 
or thirty feet. At one point the dip of the beds seemed to be westerly 
at 70°, at another point the beds appeared to be vertical; the strike, 
however, was pretty constant and uniform, and showed the above 
difference of about 30° between that of the beds and that of the cleav¬ 
age, but as the observations we're taken in separate spots they are not 
of much value. When the cleavage was well marked, the bedding lines 
were obliterated; where the bedding could be determined by the occur¬ 
ence of grit bands, there appeared to be no cleavage, or, if there were, it 
coincided with the bedding. These little difficulties are of common 
occurrence. 
In the lower ground to the west of the slate ridge are large quar¬ 
ries opened for the purpose of getting a granitic rock of the kind which 
I propose to call Elvanite. One band of this, in which there are two 
quarries opened, appears to be at least twenty yards wide, and to run 
about N.N.E. in the strike of the beds; but on the west side of it, in the 
lower part of the quarry, three or four smaller and rather irregular veins, 
. of ten to twenty feet only in width, run in a more easterly and westerly 
direction about E. 35 N. or W. 35 S., as if they branched out of the 
larger mass, a point which further excavation was required to determine. 
The slate near these veins was slightly altered, much shattered, and 
stained of a dark-brown colour, but this alteration could not be per¬ 
ceived at a greater distance than a few feet from the dyke, and at ten 
yards from it the slate was quite unaltered. The smaller granite veins 
were composed of a pale-yellow compact or finely granular rock, some- 
