256 



Mariin's Geological Memoir, and 



Our next figure (ll^.) is more complex. It represents a 

 modification of the last, having the crest removed or scooped 

 out, apparently by the subsequent agency of denudation, which 

 process has laid bare some of the inferior beds. These upland 

 valleys are of the class to which the name of valleys of elevation 

 has been assigned by Dr. Buckland, from the circumstance of 

 the strata having been apparently heaved upwards by internal 

 force, prior to the removal of the fractured materials in the 

 cavity, and before the edges of the broken strata were rounded 

 or modified by diluvial currents. 



In those which are simply valleys of denudation or erosion, 

 the drainage passes longitudinally along their centres ; but in 

 those of elevation, as will be more particularly described, the 

 drainage is effected by lateral openings or fissures in their 

 escarpments, forming transverse valleys, at right angles to the 

 direction of the longitudinal valleys. 



The authors of the Outlines notice many instances of this 

 configuration of surface, in the circuit of a few miles round 

 Bristol. 



Mountain Limestone^ 



Old Bed Sandstone. 



Mountain Limestone. 



The contour of this figure has a general resemblance to the 

 arrangement of the Weald, with which it must be classed. 

 The drawing of the latter is necessarily on a more distorted 

 scale, as it comprises an 115 



extent of many miles. Many 

 others which traverse the 

 chalk range, in various parts 

 of the island, are described 

 to present a similar combin- 

 ation of circumstances. 



In the masterly description of the south-western coal field 

 of England, to which we have referred, it is shown that this 

 district is made up of a number of hasins or hollows of less or 

 greater extent, accordingly as the undulations of the inclined 

 strata are more or less rapid ; such strata dipping fi-om every 

 point in the circumference of each basin towards its centre. 

 In their map of the district, the authors adopted a mode of 

 describing the boundaries of these basins by what they term 



