Ch. VII] ALLUVIUM. 79 



We have an opportunity of seeing in the Bermuda islands the niannei 

 in which the waves of the Atlantic have worn, and are now wearing out, 

 deep smooth hollows on eveiy side of projecting masses of hard limestone. 

 In the annexed drawing, communicated to me by Capt Nelson, R. E., the 

 excavations c, c, c, have been scooped out by the waves in a stone of very 

 modern date, which, although extremely hard, is full of recent corals and 

 shells, some of which retain their color. 



When the forms of these horizontal grooves, of which the surface is 

 sometimes smooth and almost polished, and the roofs of which often 

 ■overhang to the extent of 5 feet or more, have been carefully studied by 

 geologists, they will serve to testify the former action of the waves at 

 innumerable points far in the interior of the continents. But we must 

 learn to distinguish the indentations due to the original action of the sea, 

 and those caused by subsequent chemical decomposition of calcareous 

 rocks, to which they are liable in the atmosphere. 



I shall conclude with a warning to beginners not to feel surprise if they 

 can detect no evidence of the former sojourn of the sea on lands which 

 we are nevertheless sure have been submerged at periods comparatively 

 modern ; for notwithstanding the enduring nature of the marks left by 

 littoral action on calcareous rocks, we can by no means detect sea-beaches 

 and inland cliffs everywhere, even in Sicily and the Morea. Gn the con 

 trary, they are, upon the whole, extremely partial, and are often entirely 

 wanting in districts composed of argillaceous and sandy formations, which 

 must, nevertheless, have been upheaved at the same time, and by the same 

 intermittent movements, as the adjoining calcareous rocks. 



CHAPTER VII. 



ALLUVIUM. 



Alluvium described — Due to complicated causes — Of various ages, as shown in 

 Auvergne — How distinguished from rocks in situ — Sand-pipes in chalk — Allu- 

 vial terraces caused by oscillations in the level of land. 



Between the superficial covering of vegetable mould and the subjacent 

 rock there usually intervenes in every district a deposit of loose gravel, 

 sand, and mud, to which the name of alluvium has been applied. The 

 term is derived from alluvio, an inundation, or alluo, to wash, because the 

 pebbles and sand commonly resemble those of a river's bed or the mud 

 and gravel washed over low lands by a flood. 



A partial covering of such alluvium is found alike in all climates, from 

 the equatorial to the polar regions ; but in the higher latitudes of Europe 

 and North America it assumes a distinct character, being very frequently 

 devoid of stratification, and containing huge fragments of rock, some an- 

 gular and others rounded, which have been transported to great distances 

 from their parent mountains. When it presents itself in this form, it has 

 been called " diluvium," " drift," or the " boulder formation ;" and its prob- 



