336 Geological Society. 



the fault-intrusion, it is concluded that they are contemporaneous, 

 and that the uprise of the magma may be considered as comple- 

 mentary to the subsidence. The Glen Coe subsidence is compared 

 with the subsidence which took place in the Askja caldera in 

 Iceland in 1875. 



It is considered probable that the lobe of the Cruachan granite, 

 which invaded the sunken area of Glen Coe, was admitted by a 

 further subsidence of part of the rock-mass within the cauldron, 

 and that the granite occupied the cavity thus formed. 



A theory is advanced that the Cruachan granite -mass also 

 originated in a subsidence of the schists in place of which the 

 granite is now found, the magma welling up the sides of the sinking- 

 mass and rilling in the subterranean cauldron. Evidence for this 

 is adduced in the form of the intrusion ; in the presence of a curved, 

 flanking fault ; and in the shearing of earty consolidated parts of 

 the mass. 



The dykes point to the operation of regional tensional stresses 

 which, co-operating with the pressure of the magma, opened parallel 

 north-north-east and south-south-west fissures. It is suggested 

 that the concentration of the dykes was due to the pasty condition 

 of the internal parts of the granite-mass, which yielded to the 

 .stresses and caused a localization of fissures in the surrounding 

 solid rocks. 



After the intrusion of the majority of the dykes, a further sub- 

 sidence within the Cruachan granite-mass admitted the central 

 core of the Starav granite. 



The principle underlying the interpretation of the phenomena 

 described is the upward movement of igneous magmas in correlation 

 with complementary subsidence of portions of the earth's crust. 



2. ' The Pitting of Flint-Surfaces.' By Cecil Cams- Wilson, 

 F.R.S.E., F.G.S. 



Kegular pittings of uniform size are occasionally seen on flints 

 which have been exposed to the weather. They have been referred 

 to by various authors, but no satisfactory explanation of their 

 origin has been given. The author procured some interesting 

 examples occurring in a recent deposit near Folkestone. This 

 deposit is formed of materials which appear to have been washed 

 down from the adjacent chalk-hills. The flints appear to have 

 been derived from the sandpipes in the Chalk : their surfaces arc 

 much decomposed. The removal of the colloid silica has rendered 

 them very porous, and they absorb a good deal of water. It is 

 believed that the pittings are due to mechanical action. Obser- 

 vations and experiments carried out by the author indicate that 

 such markings cannot have been produced by blows, nor by any 

 process of desiccation, and that the freezing of the absorbed water 

 seems to be the only satisfactory explanation to account for the 

 various details of the phenomenon. 



