ANDERSON : THE VOLCANOES OF ICELAND. 
169 
or yards, but one of these at Thingvalla, more in the centre of the 
island, the Allmanagia, has a throw of about 100 feet. In this case, 
the author is satisfied that the gid is due to the unequal settling of a 
crust of lava formed on the surface of a still fluid mass, which had 
found an outlet and flowed out after the solidification of the surface. 
He is not prepared however to say that this explanation will hold 
good in the case of all the rifts on the Reykjanees peninsula. It 
certainly would not in the case of the great fissure from which the 
Skapta lava was erupted. Consequently any clear case of the forma- 
tion of a new gid in strata long cooled and solidified would have been 
well worth investigation. 
From a careful examination of the locality it appeared that no 
fresh formation of a gid has taken place, but that certain small 
portions of the rock on which the lighthouse stands had been loosened 
partly by ordinary denudation and partly by earthquakes, which are 
frequent here, and had fallen on to the beach. The strata of partly 
consolidated volcanic ashes and lava are quite continuous at the end 
of the small cove or recess between the two large rocks above referred 
to. 
