456 - - DYNAMICAL GEOLOGY. [Boox III: | 
with long holes by the teredo; while wooden piers, and the bottoms 
of wooden ships, are often rapidly perforated. Saxicavous shells, 
by piercing stone and leaving open cavities for rain and sea-water to 
fill, promote its decay. 
4, Many animals exercise a ruinously destructive influence upon 
vegetation. Of the various insect plagues of this kind it will be 
enough to enumerate the locust, phylloxera, and Colorado beetle. 
The pasture in some parts of the south of Scotland has in recent 
years been much damaged by mice, which have increased in numbers 
owing to the indiscriminate shooting and trapping of owls, hawks, 
and other predaceous creatures. Grasshoppers cause the destruction 
of vegetation in some parts of Wyoming and other Western Territories 
of the United States. The way in which animals destroy each 
other, often on a great scale, may likewise be included among the 
geological operations now under description. 
§ 2. Conservative Action. 
Plants.—The protective influence of vegetation is well known. 
1. The formation of a stratum of turf protects soil and rocks from 
being rapidly removed by rain or wind. Hence the surface of a 
district so protected is denuded with extreme slowness except along 
the lines of its water-courses. é 
2. Many plants, even without forming a layer of turf, serve by 
their roots or branches to protect the loose sand or soil on which 
they grow from being removed by wind. The common sand-carex 
and other arenaceous plants bind littoral sand-dunes and give them 
a permanence which would at once be destroyed were the sand laid 
bare again to storms. In North America the sandy tracts of the 
Western Territories are in many places protected by the sage-brush 
and grease-wood. ‘The growth of shrubs and brushwood along the ~ 
course of a stream not only keeps the alluvial banks from being so 
easily undermined and removed as would otherwise be the case, but 
serves to arrest the sedimeut in floods, filtering the water, and thereby 
adding to the height of the flood plain. On some parts of the west 
coast of France extensive ranges of sand-hills have been gradually - 
planted with pine woods, which, while preventing the destructive 
inland march of the sand, also yield a large revenue in timber, 
and have so influenced the climate as to make these districts a 
resort for pulmonary invalids.’ In tropical countries the mangrove’ 
grows along the sea-margin, and not only protects the land, but 
adds to its breadth, by forming and increasing a maritime alluvial 
belt. 
3. Some marine plants likewise afford protection to shore rocks. 
This is done by the hard incrustation of calcareous nullipores; like- 
* De Lavergne, “ Economie rurale de la France depuis 1789,” p. 297. Ldin. Review, 
Oct, 1864, article on Coniferous Trees. 


