438 MR. B. HOBSON ON THE IGNEOUS ROCKS 



normal horizontal position overlies the agglomerate. A few yards 

 to the N.W. of the mass 525, where the lava-sheet is much disturbed, 

 a small mass of lava is overlain by agglomerate ; but this is perhaps 

 due to inversion, as it seems to be the rule that lava overlies 

 agglomerate. 



On the north-eastern side of the great tilted bed, where the lava 

 has been removed by denudation, we have the underlying agglo- 

 merate exposed. It is ver}^ coarse and contains blocks of augite- 

 porphyrite as much as 4 X 3 feet and of black Poolvash marble, one 

 of which measures 4 x !■! X 2 feet. 



Large patches of limestone, apparently belonging to the Poolvash 

 marble, occur in the agglomerate here. One measures 20 x 9 x 4 

 feet, and another near low-water mark 53x24 feet. Whether 

 they have been deposited in situ or not is hard to say, they are so 

 involved in the agglomerate. 



About 320 yards to the west of the great tilted lava-bed and S.S.E. 

 of the " Remains of Burial Ground " before mentioned, a bed of 

 augite-porphyrite dipping at from 30° to 40° to N. 84° E., and 

 varying from 1 foot 3 inches at its landward end to 7 feet in thickness 

 seawards, may be observed in the tuff. This bed, like that [174] 

 previously mentioned, owes its present position to a fault striking 

 in this case N. 6° AY. 



Proceeding northward along the east coast of Bay-ny-Carrickey we 

 observe the volcanic tuff and the black Poolvash marble interstra- 

 tified. This is particularly well seen to south of Close-ny-Chollagh 

 Point, opposite the " H " of " High Water Mark," on the six-inch 

 map (sheet 16) *. This shows that the tuff is at least in part of 

 Lower-Carboniferous age, and since the dip of the tuff where it can 

 be observed is usually westerly, we may conclude that the augite- 

 porphyrite dykes and lava-beds at Scarlet are not newer, but 

 probably belong to the same period. 



A dyke, apparently belonging to the augite-porphyrite series, 

 occurs at Langness. The locality is near " The Arches " in the 

 little cove immediately to north of the most northerly natural arch. 

 The dyke is 3 feet wide, strikes jN". 60° E., and cuts through both 

 the Silurian slates and two isolated patches of the Carboniferous 

 conglomerate. The rock is marked by numerous bands of amygdules 

 showing fluxion-structure. Under the microscope it is seen to be 

 full of minute felspar-laths and larger, perhaps porphyritic, felspars, 

 while porphyritic pseudomorphs, probably after olivine, are embedded 

 in a base darkened by abundant iron-ore. 



V. Macroscopical and Microscopical Characters of the Rocks. 

 (a) The Diabase Series. — Not having worked out this series in 

 detail, I confine myself to describing a single specimen. No. 521. 

 My specimen is from Gullet Buigh, on the east coast of Langness. 

 The same mass appears to extend for many yards in the direction 

 N. 44° E., and is probably a dyke. 



The rock somewhat resembles the greenstones at Mynydd Gader, 

 * See also Home, Trans. Edin. Geol. See. vol. ii. (1874) p. 333. 



