T. Mellard Reade — On Miniature Sand-domes. 



21 



external form of the boss or tumescence. The perfect regularity 

 and smoothness of these protuberances was very remarkable ; they 

 appeared to be in all cases segments of spheres connected with the 

 plane of the shore by reflex spherical curves. The largest I dis- 

 sected measui'ed 3 inches in diameter rising half an inch. The 

 thickness of the dome was | an inch and the cavity below was 3 inch 

 diameter also. The following: is a section — 



Clean, fine, siliceous sand seems d priori a very unlikely material 

 for blisters to arise in, yet there they were. Occasionally the 

 crown of the dome had little cracks across it as if broken in the 

 extension or swelling up of the dome of sand. 



Further examination showed that the sand in question lay at the 

 foot of a little line of cliff eroded in the Peat and Forest Bed. It 

 rested upon a bed of blue clay, usually bare, and is about a foot thick. 

 The bosses were found along a zone of the sand running parallel to 

 the peat cliff and from 6 to 10 feet from it. I came to the conclu- 

 sion that these little domes were lifted by air imprisoned between 

 the clay and the moist surface of the sand, the water collecting on 

 the surface of the peat percolating from the line of cliff along the 

 clay, and forcing the interstitial air upwards. The cohesion of the 

 moist surface layer of sand being sufficient to prevent its escape at 

 once upwards, it has forced itself along a line of laminee laterally, 

 and so assumed the form of a flat bubble. The grains of sand form 

 the voussoirs of the dome and the water the cementing material. 

 Once formed, they will stand without the impi'isoned air so long as 

 the sand is moist. I have often observed blow-holes in the shore 

 sand and air issuing with a hissing noise, and I have also seen minute 

 bubbles in the shore formed by a film of sand cemented by water, 

 but never anything like what I now describe. 



Probably very few people know how much interstitial air space 

 there is between grains of sand. I have just made an experiment 

 to ascertain it, I filled a bottle with 7 oz. of perfectly dry sand 

 shaking it well together. I then immersed it in a vessel of water with 

 the orifice below the surface. Bubble after bubble arose until the point 

 of saturation was reached, but the bulk of the sand remained con- 

 stant, neither increasing nor diminishing. On weighing it, I found it 

 held 2 oz. of water. Taking the specific gravity of the quartz 

 grains at 2-6, the relative bulks were : sand 7, water 5. That is, out 

 of 12 cubic feet of sand and water at the point of saturation of the 

 sand, 7 would be quartz and 5 water. Thus in saturating 12 feet of 

 absolutely dry sand (which I need hardly say is never met with in 



