Ch. XI.] BRITISH ALLUVIUMS. 147 



been caused by earthquakes or excavated by the power of 

 running water during the rise of the land. The alluvium in 

 those more modern valleys would consist partly of pebbles 

 washed out of the older gravel before mentioned, but chiefly of 

 fragments derived from the wreck of those rocks which were 

 removed during the erosion of the valleys. 



Many of the most widely distributed of the British allu- 

 viums may we think be referred to the action of the sea 

 previous to the elevation of the land ; and for this reason we 

 never expect to be able to trace all the pebbles to their parent 

 rocks. If it be objected that the high antiquity thus ascribed 

 to many of our superficial deposits seems inconsistent with their 

 actual state of preservation, we may observe, that they are 

 often composed of indestructible materials, such as flint and 

 quartz, and in many cases they have been protected for ages 

 from the corroding action of the atmosphere by an envelope of 

 loam or clay, from which they have been partially and slowly 

 washed out by rain. 



It must not; however, be understood that we refer the 

 greater part of the alluviums scattered over our continents to 

 the waves and currents of the sea, but merely some of those 

 which have been justly regarded as most singular and anoma- 

 lous, both in position and in the discordance of their contents 

 with any known rocks in the adjacent countries. 



Grooved surface of rocks. We sometimes find the surface 

 of large tracts hollowed out extensively in parallel grooves, 

 such as have been described by Sir James Hall on the summits 

 of the Corstorphine Hills, where I have myself examined them, 

 in company with Dr. Buckland. These grooves may have 

 been caused by the friction of blocks rolled along the floor of 

 the ocean before the country emerged from the deep. The 

 same appearances may be seen on a smaller scale, in the beds 

 of many mountain- streams in Scotland, and I observed them 

 strikingly displayed on Etna, in the defile called the Portella 

 di Calanna, where a hard blue lava of modern date has been 

 furrowed in this manner by the rolling of blocks down a steep 

 declivity. I< 2 



