252 Hobbs — Features Formed at the Time of Earthquakes. 



explained the dikes by the upward movement of sand in fis- 

 sures generally at the time of earthcpiakes. 



Any consideration of the mounds found within the Gulf 

 Plain should take account of the "mud lumps" which are 

 constantly forming in the Mississippi delta. These hillocks 

 make their appearance as small isolated cones of mud which 

 often appear above tide and have craters at their summits 



Fig. 4. Large sandstone pipe shown in cliff in limestone, Anglesea. 

 (Photograph by Edward Greenly, Esq.) 



from which a spring of dirty water issues. Later in their his- 

 toiw their conical form is lost and the water issues from their 

 slopes. This water is often loaded with salt, oxide of iron, or 

 lime carbonate, and carries mud and sand. Marsh gas, carbon 

 dioxide, and nitrogen are also given off from them in quantity. 

 Lyell states* that the tubular cavities up which the springs 



* Lyell, Principles of Geology, vol. i, pp. 443-449. E. Eeclus, New Phys- 

 ical Geography, vol. i, pp. 211-212, 1894. 



