396 Tertiary Formations. paet n. 



feet long by two and a half in width : the double lines 

 represent the fissures partially filled with oxide of iron 

 and agate : the curvilinear lines show the course of the 

 innumerable, concentric, concretionary zones of different 

 shades of colour and of coarseness in the particles of 

 tuff. The symmetry and complexity of the arrangement 

 gave the surface an elegant appearance. It may be 

 seen how obviously the fissures determine (or have been 

 determined by) the shape, sometimes of the whole con- 

 cretion, and sometimes only of its central parts. The 

 fissures also determine the curvatures of the long undu- 

 lating zones of concretionary action. From the varying 

 composition of the veins and concretions, the amount 

 of chemical action which the mass has undergone is 

 surprisingly great ; and it would likewise appear from 

 the difference in size in the particles of the concretionaiy 

 zones, that the mass, also, has been subjected to internal 

 mechanical movements. 



In the peninsula of Lacuy, the strata over a width 

 of four miles have been upheaved by three distinct, and 

 some other indistinct, lines of elevation, ranging with- 

 in a point of north and south. One line, about 200 

 feet in height, is regularly anticlinal, with the strata 

 dipping away on both sides, at an angle of 15°, from a 

 central ' valley of elevation,' about 300 yards in width. 

 A second narrow steep ridge, only sixty feet high, is 

 uniclinal, the strata throughout dipping westward; 

 those on both flanks being inclined at an angle of from 

 ten to fifteen degrees ; whilst those on the ridge dip in 

 the same direction at an angle of between thirty and 

 forty degrees. This ridge, traced northwards, dies away ; 

 and the beds at its terminal point, instead of dipping 

 westward, are inclined 12° to the north. This case 



containing incoherent matter is singular ; D'Aubuisson (' Traite de 

 Geogn.' torn. i. p. 318) remarks on this circumstance. 



