Vol. XIIL] 
[Part III. 
PROCEEDINGS 
OF THE 
YORKSHIRE 
GEOLOGICAL AND POLYTECHNIC SOCIETY. 
Edited by \V. LOWER CARTER, M.A., F.G.S., 
AND WILLIAM CASH, F.G.S. 
18Q7- 
NORWEGIAN RHOMB-PORPHYRIES IN THE HOLDERNESS BOULDER-CLAYS. 
BY ALFRED HARKER, M.A., F.G.S. 
{Read October '29th, 1896.) 
[Abstract.] 
Among the various rocks of Scandinavian origin, boulders of 
which occur in our east-coast boulder-clays, and have been utilised 
by Glacial geologists in tracing the movements of the Pleistocene ice, 
none are better adapted for this purpose than the rhomb-porphyries. 
Not only have they a striking and easily-recognized characteristic 
in the peculiar crystallographic habit of their porphyritic felspars, 
which causes them to present rhombic sections on a face of the rock, 
but in this and other peculiarities they are unique or almost unique"^ 
among known rocks. They are thus among the few boulders which 
can be set down at a glance and without fear of error as having 
their source in a defined foreign area. 
Although this conclusion seems beyond doubt, it may, perhaps, 
be thought to be strengthened by the cumulative evidence derived 
from the identification among our boulders, not only of rhomb- 
porphyries, but of particular varieties of rhomb-porphyries, which 
* Osann has described rhomb -porphyries from Texas. 
A 
