76 Mr. Mallet on a hitherto unobserved Structure 



occurrence of limestone imbedded in trap is unusual. Kirwan, however, mentions 

 that the " variolites" of Drac contain rounded masses of limestone and steatite. 



Considerable excavations for a new dock, covering eight acres, are now in 

 progress at Galway, and afford ample opportunity of examining the intimate 

 structure of the trap rock. Its general surface, where laid bare, is about twelve 

 feet above low water mark ; it is rough, vesicular, and apparently water-worn, and 

 rises every where into irregular " hummocks." The mass of the rock consists 

 of greenstone of a dark leek-green colour, passing into purplish gray ; rather fine- 

 grained, and softer than usual, yielding, with difficulty, a whitish streak with 

 steel. It absorbs water slightly, and becomes very dark-coloured when wet ; is 

 sonorous when in thin pieces; and has an average specific gravity of 2.87; its 

 fracture is uneven, and sometimes imperfectly conchoidal. Its texture and colour 

 are however extremely various, veins of several different constituents continually 

 occurring in it, mixed and contorted in the most capricious manner — a single 

 hand specimen often containing red granite and greenstone, passing into and 

 varied in every possible way by hornblende, augite, schorl, albite, felspar, 

 olivine, &c. Nearly in the centre of the surface of the rock exposed by excava- 

 tion, there occurs a great vein of white hornstone rising with its laminae vertical, 

 and in something of a pyramidal form, in the midst of the trap. Its structure is 

 lamellar, or pseudo-crystalline, with some perpendicular rifts ; its substance is 

 perfectly uniform, containing no imbedded minerals ; its texture very hard and 

 porcellaneous, with a high specific gravity ; the longitudinal fracture slaty, and 

 cross fracture splintery. At the surfaces of contact it is accurately moulded to 

 the trap, but no where adherent to it. It appears to have been ejected after, and 

 through the dyke itself, and is probably formed from simpler rocks, possibly slate, 

 at a much greater depth.* 



The minerals found imbedded in this trap-dyke are many and various. The 

 following have been already collected, and others probably remain for future 

 explorers : — 



Mica, — brown and white, rarely. 



Felspar, — in brown, red, and white crystals. 



Albite, — large crystals in sienite, and druses in the hornstone. 



* This hornstone lias since been found to afford an excellent substitute for the costly " Turkey 

 whetstones." 



