CHAP. I. 



Calcareous Deposit. 



basaltic bases, with numerous crystals of augite, horn- 

 blende, olivine, mica, and sometimes glassy feldspar. 

 A common variety is almost entirely composed of crystals 

 of augite with olivine. Mica, it is known, seldom occurs 

 where augite abounds ; nor probably does the present 

 case offer a real exception, for the mica (at least in my 

 best characterised specimen, in which one nodule of this 

 mineral is nearly half an inch in length,) is as perfectly 

 rounded as a pebble in a conglomerate, and evidently 

 has not been crystallised in the base, in which it is now 

 inclosed, but has proceeded from the fusion of some pre- 

 existing rock. These compact lavas alternate with tuffs, 

 amygdaloids and wacke, and in some places with coarse 

 conglomerate. Some of the argillaceous wackes are of 

 a dark green colour, others, pale yellowish-green, and 

 others nearly white ; I was surprised to find that some 

 of the latter varieties, even where whitest, fused into a 

 jet black enamel, whilst some of the green varieties 

 afforded only a pale gray bead. Numerous' dikes, con- 

 sisting chiefly of highly compact augitic rocks, and of 

 gray amygdaloidal varieties, intersect the strata, which 

 have in several places been dislocated with considerable 

 violence, and thrown into highly-inclined positions. 

 One line of disturbance crosses the northern end of 

 Quail Island, (an islet in the bay of Porto Praya) and 

 can be followed to the mainland. These disturbances 

 took place before the deposition of the recent sediment- 

 ary bed ; and the surface, also, had previously been 

 denuded to a great extent, as is shown by many trun- 

 cated dikes. 



Description of the calcareous deposit overlying the 

 foregoing volcanic rocks. — This stratum is very con- 

 spicuous from its white colour, and from the extreme 

 regularity with which it ranges in a horizontal line for 

 some miles along the coast. Its average height aDove 



