98 PALEOZOIC TIME. [CHAP. VI. 



action is such, that if they were allowed full play, and were 

 not checked or balanced by uplifting movements, every 

 continent would gradually be reduced in height, and worn 

 down to a level but little above that of the sea, while the 

 surrounding waters would be choked and shallowed by the 

 materials poured into them from the wasting land. It 

 would appear, therefore, that the Carboniferous was a period 

 when this theoretical result was more nearly approached than 

 it ever has been before or since ; when the continents were 

 gradually lowered by the combined action of detrition and 

 depression, the area of high ground being continually dimi- 

 nished, but the area of low-lying, swampy ground at or about 

 the sea-level being continually increased, so that at the close 

 of the period similar physical conditions as to climate, rain- 

 fall, surface slopes, soil and vegetation, seem to have pre- 

 vailed throughout the greater part of the northern hemi- 

 sphere. 



Such a condition of affairs will account for the remark- 

 able uniformity of life which prevailed throughout the 

 Carboniferous period, a fact to which attention was called 

 on p. 89. The geographical changes being so slight, and 

 during the earlier portion of the period consisting chiefly 

 in an extension of the marine areas at the expense of the 

 land, we may suppose that the seas retained their same 

 relative positions and connections throughout an enormous 

 lapse of time, and that the denizens of these seas spreading 

 far and wide through the broadening water-spaces found 

 everywhere a similarity of conditions that enabled them to 

 nourish and survive without change or variation. Later 

 on, when the seas were shallowed by the continued depo- 

 sition of material derived from the continents, and when 

 the higher land was everywhere encircled by a wide belt 

 of low-lying jungle and swampy ground, intersected by 

 sluggish water-ways and lagoons, the conditions would be 

 exactly those where nature would present a monotonous 



