4 The Geological Structure 



exposed, and between tlieni are bands of brown ripple-marked 

 sandstone and volcanic tufa. The whole dip west at an angle of 

 15°. The amygdaloids are evidently superficial lava currents, 

 presenting in some places those pipe-like cavities described by 

 Sir W. E. Logan in his account of this place, and which must have 

 been caused by air bubbles rising through the superficial molten 

 mass. The amygdaloid is much, more vesicular above than be- 

 low, and its cavities and veins are filled with agate, ciystalline 

 quartz, calc spar, and flesh-coloured laumonite. 



The shore for some distance follows the strike of these beds, 

 in whicb the waves, acting on the tufa and mineral veins, have 

 excavated many small caverns and ravines. Some of these exca- 

 vations are at a little higher level than that of the waters of the 

 lake at present : and they are very instructive in the explanations 

 which, they aftbrd of erosions observed even on the summits of 

 the hills. 



Five miles westward of Anse aux Crepes, the ledges of the 

 coast are broken across, probably along the line of a transverse 

 fracture of the beds, to form the little bay of Maimanse. On 

 tbe east side of this bay, we find another section of trappean and 

 sedimentary rocks, apj)arently a little lower in the series than 

 those of Anse aux Crepes. The highest bed of trap is amygda- 

 loidal above, and more compact below, where it rests upon a 

 brown conglomerate with syenitic pebbles, and thin layers of 

 l>rown sandstone. The latter consists of grains, often rounded, 

 of quartz, felspar, and hard black slate, stained by peroxide of iron, 

 and cemented by carbonate of lime, which also enters into the 

 cement of tbe conglomerate. The conglomerate rests upon another 

 bed of trap, wbicb in its upper part is largely amygdaloidal, and 

 contains small agates. It also holds syenitic fragments, probably 

 mixed witb the scoriacous matter of its surface, at the time when 

 the conglomerate was deposited above, so that, as is ofren seen in 

 such cases, the upper part of the trap passes into the cono-lomerate- 

 These rocks present no appearance of igneous alteration subse- 

 quent to their deposition, and dip S. 70° W. 35°. 



At the head of the bay, and at its western side, the sections 

 show alteiTiations of compact and amygdaloidal trap and hard- 

 ened volcanic ash, in very regular layers ; and holding numerous 

 veins of Calc Spar, Laumonite and Quartz, with small quantities 

 of Epidote, Prehnite, Sulphurets and Carbonates of Copper, na- 

 tive Copper, native Silver, and Galena ; the mode of occurrence of 



