70 Mr. Caldcleugh on the Geology of Rio de Janeiro. 



ries close to the city afford great varieties of the former rock. Although its 

 fitness as a building'-stone varies considerably, it did not appear that a pre- 

 ference existed in favour of any particular quarry. Proximity seemed the main 

 object. These excavations presented favourable opportunities for observing 

 the stratification. 



The leading direction of the gneiss appeared to be N.N.W. or N.W., and 

 the dip varied considerably; in one case it was as much as 46°_, in others merely 

 a gentle inclination. The strata in one quarry were disturbed and contorted 

 in a most singular manner without the disturbing cause being apparent. 



This gneiss formation is all more or less intersected by granite veins, vary- 

 ing in thickness from two or three feet to as many inches. Some of the veins 

 were stained with oxide of iron which had extended to some distance in the 

 walls of the mass of gneiss. There was nothing to lead to a supposition that 

 they were formed anterior or posterior to the mass which they intersected. 

 From the mutual intersection of the crystallization, one would be rather led to 

 infer, at least in some instances, that they were contemporaneous. 



The only superjacent beds I met with were one of greenstone near the Va- 

 longo, and one of clay iron-stone near the Queen's House. The gneiss, al- 

 though in some spots resembling a micaceous slate, speedily recovered its 

 former aspect. 



The greenstone bed in the gneiss at the Valongo was about twelve feet 

 thick, dipping 35° in the direction of S.S.E., the rock on which it rested having 

 a slight dip to the N.W. This bed is of inconsiderable extent, and is formed 

 of globular distinct concretions, which on being struck with the hammer sepa- 

 rated in successive coatings abounding in oxide of iron. 



The clay iron stone rested on the gneiss in very considerable thickness, but 

 the nature of the ground and the proximity of the Queen's residence prevented 

 a complete examination*. 



The elevated peak called the Corcovado is situate on the Botafogo side of 

 the city, and rises 2100 English feet above the level of the sea. The summit, 

 composed of coarse porphyritic gneiss, of a hardness superior to that forming 

 the mass of the hills below, is intersected by veins of quartz and felsparf. 



In the gneiss of Rio de Janeiro, a great proportion of which may be termed 

 porphyritic, the beauty and variety of the felspar is very striking. Some of 

 the specimens of this mineral even approach to the moonstone of Cey- 

 lon, and others would no doubt furnish excellent kaolin. The decomposing 

 variety is used by the lapidaries of the city to give a polish to amethysts. In 



* The specimen. No. 37, is from this bed. 



t The large made of felspar, No. 34, is from this locality. 



