Ch. XXIX.] CENTRAL VALLEYS. 051 



150 feet deep, is a plain about 500 feet in diameter, having a pond 

 in the middle, towards which the plain slopes gently from all sides. 

 Such ponds are often seen in the interior of extinct craters. Except 

 in the middle it is shallow, and supports aquatic plants. Many leaves 

 must also be blown into it from the surrounding heights when high 

 winds prevail, so that a mass of peaty matter convertible into lignite 

 may collect here. 



Had streams of lava descending from greater heights entered this 

 Lagoa crater, they would have formed dense masses of compact rock 

 cooling slowly under great pressure, like those now incumbent on the 

 impure lignite of S. Jorge. The dip of the latter cannot be clearly 

 determined, since it is exposed to view for too short a distance ; and 

 the same may be said of the leaf-bed, part of which may be traced 

 lower down the ravine. It seems, however, to dip to the north or 

 towards the sea conformably with the general inclination of the ba- 

 saltic and tufaceous strata. 



A deep valley, called the Curral (b, fig. 708), surrounded by preci- 

 pices from 1500 to 2500 feet high, and by peaks of still greater ele- 

 vation, occurs in the middle of Madeira. It has been compared by 

 some to a crater or caldera, for its upper portion is situated in the 

 region where dikes and ejectamenta abound. The Curral, however, 

 extends, without diminishing in depth, to below the region of numer- 

 ous dikes, and it lays open to view all the beds r, s, fig. TO 6. Nor 

 do the volcanic masses dip away in all directions from the Curral, as 

 from a central point, or from the hollow axis of a cone. The Curral 

 is in fact one only of three great valleys which radiate from the most 

 mountainous district, a second depression, called the Serra d'Agoa 

 (d, fig. 708), being almost as deep. This cavity is also drained by a 

 river flowing to the south ; while a third valley, namely, that of the 

 Janella, sends its waters to the north. The section alluded to (fig. 

 708), passing through part of the axis of the island in an east and 

 west direction, shows how the Curral and Serra d'Agoa, b and d, are 



West. Fi?. 70S. East. 



Section through the central region of Madeira, from East to West. 

 A. Part of the platform, called the Paul de Serra. B. Curral ; a valley, 3000 feet deep. 



O. Pico Grande. E>. The valley of the Serra d'Agoa, 



separated by a narrow and lofty ridge, c, part of which is surmount- 

 ed by the Pico Grande, before mentioned, nearly 5400 feet high. 

 There is no essential difference between the shape of these three great 

 valleys and many of those in the Alps and Pyrenees, where the val- 

 ley-making process can have had no connection with any superficial 

 volcanic action. 



