The two principal classes of agents which destroy stone have already been described.

They are - Chemical agents, consisting of acids, etc., in the atmosphere; and Mechanical agents, such as wind, dust, rain, frost, running water, force of the sea, etc.

There are other enemies to the durability of stones, which may just be glanced at, viz. -

Lichens.

Worms or Molluscs.

Lichens

In the country lichens and other vegetable substances collect and grow upon the faces of stones.

These are in many cases a protection from the weather, and tend to increase the durability of the stone. The fine rootlets spread themselves over the surface and into the interstices, covering the face from the action of wind and weather.

In the case of limestones, however, the lichens sometimes do more harm than good, for they give out carbonic acid, which is dissolved in rain water, and then attacks the carbonate of lime in the stone.

Molluscs

The Pholas dactylus is a boring mollusc found in sea water, which attacks Hmestone, hard and soft argillaceous shales, clay, and sandstones. It also attacks wood, but granite has been found to resist it successfully.

These animals make a number of vertical holes close together, so that they weaken and eventually destroy the stone.

By some it is supposed that they secrete a corrosive juice,2 which dissolves the stone; others consider that the boring is mechanically done by the tough front of the shell covering the Pholas.3 These animals are generally small, but sometimes attain a length of five inches - the softer the rock the bigger they become. The shale beds, on which was founded the quay wall at Kirkcaldy, were so perforated by Pholades that they crushed under the superincumbent pressure, and a settlement resulted.4

The most notable instance of injury done by Pholades is at Plymouth breakwater, where, in consequence of their attacks, the limestone blocks had to be replaced by granite.4

The Saxicava is another small mollusc, found in the crevices of rocks and corals, or burrowing in limestone, the holes being sometimes six inches deep. It has been known to bore the cement stone (clay-ironstone) at Harwich, the Kentish Rag at Folkestone, and the Portland stone used at Plymouth Breakwater.

1 Kankine, Civil Engineering. 2 Hartwig's The Sea and its Living Wonders. 3 Woodward's Recent and Fossil Shells. 4 Stevenson On Harbours.