TRANSACTIONS OF SECTION C. 131 



the top. Considering the first-found boulders, it is certainly a 

 curious circumstance that these are preserved within such a 

 limited area, over three miles away from their place of deriva- 

 tion. 



The striking feature of the rock is its eminently porphyritic 

 structure, combined with the abundance and large size of the 

 idiomorphic sanidines and nephelines, which shine out in tlie 

 dark base. The nephelines especially are, as far as records of 

 nepheline rocks go, of quite exceptional dimensions. On the 

 faces of the boulders at Portobello rectangular sections of the 

 mineral can be seen of over iin. in length and ^m. in width, 

 besides square and hexagonal ones up to -S-in. in diameter ; but 

 they also go down to the size of a mustard-seed. Their colour 

 on fresh fracture is a light reddish-brown, which changes to 

 dull white by weathering and decomposition, to which they are 

 very liable, as shown by the abundance of cavities left in the 

 gre}ish-white decomposition-crust of the rock. The generally 

 large size of the sanidines, though not an uncommon occurrence 

 in trachytic rocks, is also a noteworthy feature. The crystals 

 are mostly thin-tubular in the direction of the orthoaxis and 

 largely extended in that of the clinoaxis, in cases to near liin. 

 in length, whilst in the line of the vertical axis some sections 

 reach nearly fin. They very frequently form Carlsbad twins, 

 and are in the average disposed in the base in the same way — 

 i.e., parallel to the clinopinacoid — so that, on fracture of the 

 rock in this direction, one sees mostly the long and broad 

 cleavages of this face — ooP_oo(010) — and at right angles to 

 this the narrow columnar ones of the basal pinacoid — OP(OOl) 

 — and in these the Carlsbad twinning-line is often plainly 

 observable. About the middle of the faces of some of the 

 large Portobello boulders I observed also some large square 

 sections — one of over fin. length of side — which seemed to 

 have a diagonal twinmug-line, indicating twins of the Baveno 

 type ; but as I was not successful in detaching chips of the 

 rock containing them, for closer examination, and found no 

 twins of this type amongst the small crystals in thin sections, 

 I am doubtful on this point. Eegarding the relative abundance 

 of the crystals of sanidine and nepheline, it is not easy to 

 ■determine ; in parts of the rock the former, in others the latter, 

 appear in greater number : still, I think that in the average the 

 sanidines are more abundant. 



The dark base, which occupies in parts of the rock plainly 

 less space than the crystals of the above two minerals, shows 

 on examination by the unarmed eye numerous inclusions of 

 dark minerals, some of which can be recognised by their 

 cleavage as hornblende ; but, as to the character of the re- 

 mainder, even by using the pocket lens no definite opinion can 

 be formed : one can only see that most have glassy lustre, 



