192 Analysis of Scientific Books. 



ticing in the former the enormous veins of feldspar which inter- 

 sect in all directions the strata of gneiss, of which the whole 

 island is composed. He regards the two minerals as con- 

 temporaneous. The strata run from north-east to south-west. 

 He observed in the southern part of Coll, three veins of basalt 

 in the gneiss ; two of which cut the strata obliquely, and the 

 third is parallel to them. The gneiss has experienced no 

 change whatever near its point of contact with the basalt. One 

 of the oblique veins presents a phenomenon not uncommon ; 

 the grain of the basalt is finer near the sides of the vein, than 

 in the middle. The two other veins assume at the junction, and 

 even to the distance of two inches from the walls, a slaty 

 aspect, dividing into thin plates parallel to the breadth of the 

 vein. The rock of the curious columns of Scaur-Eigg, 

 Mr. Necker says, has been improperly named a porphyry with 

 base of pitchstone by Professor Jameson ; the substance which 

 forms the base of this rock seems to differ essentially from pitch- 

 stone, in lustre, texture, and especially hardness. He considers 

 it as a new substance, or as one at least which has not been de- 

 scribed hitherto with its true character. He calls it lithoid 

 obsidian, to distinguish it from the vetreous variety. At the 

 foot of the porphyritic ridge of the Scaur, he found basalt in 

 tables, alternating with basalt in irregular prisms, down to the 

 sea-shore. On the coast to the south-east two great veins of 

 vetreous obsidian were observed to traverse the basalt. Nothing 

 has more the appearance of fracture and lustre of glass than 

 this rock. The junction is very sharp; a mathematical line 

 separates the obsidian and basalt, which are not confounded 

 together, and never pass into one another by insensible shades. 

 The basalt does not seem to have undergone any notable mo- 

 dification in the neighbourhood of the obsidian veins. The 

 base of the island on the east and south sides, is an earthy 

 basalt or tender wacke. Mr. Jameson asserts that the lowest 

 strata are composed of limestone and schistous clay. These 

 are of course under the wacke which alone is apparent in the 

 greater part of the island. 



We shall not follow Mr. Necker through the rest of the 

 Hebrides, especially as we possess in this country already 

 Dr. MaccuUoch's excellent work, of whose general accuracy 

 the Genevese geologist speaks in the highest terms. On his 

 return across the main land of Scotland, our author notices the 

 downs or moving sands on the coast of the southern Murray 

 Firth. The inhabitants, mistaken as to the cause of so for- 

 midable a phtjnomenon, do nothing requisite to hinder its 

 disastrous effects. They regard these masses of sand as 

 produced by extraordinary inundations of the sea, and 

 by rare and accidental subversions of the soil. But it is 

 not to so uncommon causes that we must have recourse to 



