Inquiry into the Origin of Mud Bushes in De Beefs Mine. 63 



diurnal range of temperature may possibly contribute something to 

 this result, even as it seems, at times, to have modified the action of 

 a barometric depression. 



There are some indications of a lagging, on the part of the mud 

 rushes, behind the changes of pressure, greater for large rushes than 

 for small. This same lagging, if it certainly exist, could produce a 

 rush of mud (which may have been started by a deep, but narrow, 

 depression) right under, or even following upon the next advancing 

 crest. The temperatures are apparently responsible for this anomaly. 

 [See the mud rushes of April, 1894.] 



Since we are unable to locate the spot where the mud is forming, 

 it is difficult to formulate a statement which shall define the precise 

 effect of atmospheric pressure upon it. The debris is clearly not 

 pervious to the air, because, if it were, the mud would flow out in 

 an almost uniform stream as fast as it could be formed. The only 

 effect of a depression would be to quicken the flow, and of a crest to 

 slacken it. 



An explanation which seems to meet some of the difficulties is 

 this : Suppose a stream of water to be percolating through the 

 boundary rock into the debris. If there should be no outlet beneath, 

 this would go on until the whole mass of debris, up to a certain 

 level depending on its capillary attraction, would be quite saturated. 

 But if the mass of wet debris be in communication, on its under 

 side, with the atmosphere, will it run out? Not unless the 

 algebraic sum of the forces within the mass downwards is 

 greater than the pressure of the atmosphere against it, upwards. 

 It is conceivable that when the head of mud is great the mud 

 may burst its bounds irrespective of the state of the barometer." 

 But when the head of mud is not great — and apparently this is 

 so in most cases — the normal spring of the air (Boyle's term 

 is convenient) will detain the mud in a state of equilibrium. The 

 power of the air to support a limited head of fluid is of course quite 

 familiar to any one acquainted with the structure of a mercurial 

 barometer. Should the analogy not be obvious the following simple 

 experiment, if carefully performed, will perhaps make it so. Take a 

 rough iron pot open only at the top, pack it with earth, and add 

 water until it can absorb no more. Shake it well and stand it to 

 settle. Drain off any water lying on the top, and then invert the 

 pot. In general the mud will remain in the pot. If now a small 

 hole be gently pierced through the bottom of the pot so as to admit 



* One would infer that if the head of mud were alone responsible, the mud 

 below would be simply squeezed out by the excess above until the equilibrium 

 was restored. 



