342 Scientinic Proceedings, Royal Dublin Society. 
less than fifty, while the length of the greater diameter may run 
to 1,000 feet, or even a quarter of a mile in exceptional cases. 
The accompanying sketches, Plates 21, 22, and 23, will serve to 
convey an idea of two types of contour assumed by these domes ; 
one may be called normal, while the other is a somewhat excep- 
tional conical or sugar-loaf form. 
A good idea of the normal forms may be suggested by com- 
paring them to what would be the appearance presented by 
keel-less ironclad vessels if inverted on Jand. 
Generally the rock of which these domes are composed is a 
coarsely porphyritic granite,* containing twin crystals of ortho- 
clase felspar from one to two inches long. Sometimes, however, 
the rock is a fine-grained granite with a large proportion of 
amorphous, or more correctly, perhaps, not distinctly crystalline 
felspar. 
Occasionally, in the first-mentioned variety, we find the mica 
either locally or throughout replaced by hornblende, the rock thus 
assuming the characters of a syenite. 
Foliation and bedding structures are not generally present, and 
from the absence of these and the relations which the rocks bear 
to the schists and gneisses surrounding them, it might be thought 
perhaps that they were really intruded masses of plutonic origin. 
Since, however, in some cases both structures are present, and 
there is nothing lithologically to distinguish these larger masses 
from some which occur in thin beds, alternating with ordinary 
well-defined gneisses and schists, it is probable that they are 
likewise of metamorphic origin. 
It should be added that, in some cases where there is no 
foliation, properly so called, there is to be observed a general 
parallelism of the large crystals of felspar. 
Although, in the majority of instances, these large spheroidal 
masses may appear to be solid throughout, it is not long before 
* An analysis of a specimen of it is given by the late M. H. Ormsby, L1.p., in a 
former number of the Journal of this Society. (New Series, Vol., iii., 26.) 
Silica, ‘ . 65:04 
Alumina, . L960 
Lime, : 5) (OMS 
Magnesia, - 2°48 
Potash, . - 12:60 
