436 MR. 1!. IIOBSON ON THE IGNEOUS ROCKS 



against the limestone. If tlie line of strike of the dyke be produced, 

 it will fall within the broad gap above described. Having failed to 

 find the rock there in ska, I searched for blocks of it and found one 

 there near low- water mark measuring 4 ft. x 3| ft. x 3 ft. This 

 renders it almost certain that the dyke occurs in the gap. 



On the south-western side of the gap is a large patch of limestone, 

 the lower beds of which Gumming considers to belong to the Lower 

 Limestone series which occurs to the north of the gap, while the 

 upper portion of it is held by him to belong to the Poolvash 

 limestone *. To the south the patch of limestone is cut off by 

 a wall-like dyke |517] of augite-porphyrite. This dyke is 

 probably continuous with the great dyke [500 a] to be subse- 

 quently described. On the south-western side of the dyke 

 [517] is a portion [528] of the Lower Limestone with shaly layers 

 violently contorted, being thrown into an anticlinal curve and tilted 

 so that the crown of the arch dips at 17° to jS". 66° W. Gumming 

 represents this in his section (loc. cit.) as being iii situ, but it appears 

 to be borne on the back of a mass of augite-porphyrite (perhaps part 

 of the same mass as 517) which is exposed, at low water only, to the 

 east of it, with the denuded sides of the limestone arch still overlying 

 its margins. Abutting against the western margin of this small con- 

 torted limestone-patch is augite-porphyrite, forming part of a very 

 large intrusive mass which terminates southward in the Stack of 

 Scarlet and extends nearly 200 yards north-westwards. 



The Stack of Scarlet is a somewhat precipitous, dark-coloured, 

 obtusely conical mass, rising 50 feet above low-water mark. It 

 is an island at high tide. The rock of the Stack is more or less 

 distinctly columnar. The columns are from 15 to 18 inches 

 across and nearly vertical. Some evidence of the age of the Stack 

 is afforded by the fact that on the south-west side a band of black 

 limestone about 2 inches broad is entangled between the columns. 

 Between the limestone patch [528] and the Stack are three other 

 small masses of black limestone embedded in the augite-porphyrite 

 or agglomerate. The intrusive mass forming the Stack appears to 

 be continued northwards to form a low isthmus, and then rises into 

 a great wall-like ridge [500 a] some 40 feet across. Here it has all 

 the characters of an intrusive dyke. Its sides are vertical and at 

 high tide it forms a promontory^ It can be traced north-westwards 

 " to the point where an isolated patch of porphyrite rests on the 

 ash " t. Whether it can be traced beyond the " isolated patch " is 

 uncertain, but it is perhaps continuous with a broadband of augite- 

 porphyrite which extends from near that point, protruding but 

 slightly above the grass towards a mass [503] which has almost the 

 appearance of a bedded lava-sheet rather than that of an intrusive 

 dyke. 



The space between the dyke [500 a] and the sea on its south-west 

 side is occupied by agglomerate, which can be seen at low water to 

 overlie greenish volcanic tuff. Similar agglomerate covers the space 



* ' Isle of Man,' pi. vii. fig. 5. 



t Home, Trans. Edin. Geol. Soc. vol. ii. (1874) p. 335. 



