chap. xin. Psettdo- Dikes. 439 



spathic matter, surrounding a dark translucent siliceous 

 centre, in which grains of quartz not quite blended 

 into the paste could be distinguished : this uniformity 

 in the nature of the fragments shows that they are not 

 of mechanical, but of concretionary origin, having re- 

 sulted perhaps from tho self- breaking up and aggrega- 

 tion of layers of indurated tuff containing numerous 

 grains of quartz, — into which, indeed, the whole mass 

 in one part passed. The second variety is a reddish 

 non-porphyritic claystone, quite full of spherical 

 cavities, about half an inch in diameter, each lined with 

 .a collapsed crust formed of crystals of quartz. The 

 third variety also consists of a pale purple non-porphy- 

 ritic claystone, almost wholly formed of concretionary 

 balls, obscurely arranged in layers, of a less compact 

 and paler coloured claystone ; each ball being on one 

 side partly hollow and lined with crystals of quartz. 



Pseudo-Bikes. — Some miles up the harbour, in a 

 line of cliffs formed of slightly metamorphosed tuface- 

 ous and porphyritic claystone beds, I observed three 

 vertical dikes, so closely resembling in general appear- 

 ance ordinary volcanic dikes, that J did not doubt, 

 until closely examining their composition, that they 

 had been injected from below. The first is straight, 

 with parallel sides, and about four feet wide; it con- 

 sists of whitish, indurated tufaceous matter, precisely 

 like some of the beds intersected by it. The second 

 dike is more remarkable ; it is slightly tortuous, about 

 eighteen inches thick, and can be traced for a con- 

 siderable distance along the beach ; it is of a purplish- 

 red or brown colour, and is formed chiefly of rounded 

 grains of quartz, with broken crystals of earthy feld- 

 spar, scales of black mica, and minute fragments of 

 claystone porphyry, all firmly united together in a hard 

 sparing base. The structure of this dike shows obviously 



