GEOLOGICAL SKETCH 287 



These thick Eocene beds rise to the summit of the 

 wooded hills. Viewed from a distance they seem to 

 ascend like giant steps from the foot-hills to the higher 

 mountains. Sometimes they are moulded into rounded 

 domes, but more often are abruptly carved into narrow 

 ridges that fall in a steep incline to the valley or end 

 in a bare precipitous cliff. The slopes of the hills 

 sink down into deep ravines. A small stream, rarely 

 a torrent, trickles over the valley bed. Other valleys 

 have not a trace of water. It is easy to understand 

 how these valleys are increased in width when we see 

 the debris pouring down their slopes, loosened by the 

 heat and cold, dislodged by the percolation of water or 

 split asunder by the roots of the trees. But it is more 

 difficult to appreciate the slow process by which the 

 valleys have been deepened ; how these small streams, 

 often only a trickle of water or flowing after rain for a 

 few clays in the year, can have dug a thousand feet 

 into the ravines. For so slow an agent to produce 

 so great an effect we seem to require an almost 

 illimitable time. 



We are liable to form an inaccurate conclusion as to 

 the activity of the erosion of these Tertiary rocks when 

 we view the valleys in the dry months of the year. 

 Then we see so little sign of the wear and tear of the 

 rock surface that we think the hills can never change. 

 But after the first heavy rains following a period of 

 drought another scene appears. From every gully in 

 the limestone cliffs piles of debris and large angulated 

 stones shoot down into the valley. Often they 

 obstruct the roads and take the workmen days to clear 

 away the ruin. Months of slow erosion are made 

 manifest in a single day. I have sometimes imagined 



