all Mineral Substances. 165 
occasionally commence at the middle, and 
extremities of the excavated ground at the 
same time, and meet half way ; and they ap- 
pear to proceed without any order, being 
heard in every part at the sametime. When 
in full activity, they are heard every minute, 
sometimes two or three at a time, then a 
small pause takes place; very often they go 
off in irregular succession, but most frequently 
at irregular intervals. In this manner they 
may continue for a few days or till one layer is 
separated, then they will not be heard, or only 
heard sparingly for a day or two; they will 
then recommence, and be as active as ever. 
These alternations of activity and rest conti- 
nue for-a few weeks, and then the thuds cease 
or nearly cease for a few weeks; they will 
then begin again with greater force than be- 
fore. L may remark, that the small cessa- 
tions are in consequence of the bending of 
every stratum before it separates from that 
which is incumbent on it, and the larger ces- 
sations happen, when the pillars, having been 
squeezed out to a greater surface, are just ca- 
pable of bearing the weight then upon them 
for a certain time; hence, the falling layers 
close up the space to the firm strata, and sus- 
pend their bending; but as the pillars very 
