292 Messrs. Buckland's and Conybeare's Observations on the 



of, fragments of mountain limestone are usually the principal ingredient in 

 the conglomerate, it seems natural to consider the dolomite in the limestone, 

 in the state of finely worn debris, as the origin of the dolomitic cement*. 



The conglomerate is remarkable, particularly in the neighbourhood of 

 Wells, for the occurrence of small irregular cavities, from an inch to a foot 

 and upwards in diameter, which are lined, and sometimes filled, with con- 

 centric plates, resembling in their structure those of an agate, and consisting 

 of calcareous spar, coarse chalcedony, and crystals of quartz. These geodes 

 are called, from their shape, Potatoe stones. They are found of smaller di- 

 mensions in the compact dolomite at Old Clevedon, and are there almost 

 wholly siliceous, being, in fact, coarse agates, hollow within, lined with 

 crystallized quartz, and sometimes containing crystals of sulphate of strontia. 

 The geodes found in the neighbourhood of Clifton have been noticed in the 

 2nd chapter. 



Besides the smaller cavities, in which the geodes have been formed, the 

 conglomerate abounds with long, low, irregular vacuities ; which usually 

 have a communication with one another, and thus extend under ground over 

 large tracts of the rock. These caverns have probably been occasioned by 

 irregular deposits of fine loose sand, which has subsequently been removed 

 from its place, and thrown out at the surface by the action of springs. Nests 

 of sand, irregularly dispersed after the manner of these cavities, are fre- 

 quently found in the interior of the conglomerate. 



Any one of these caverns, in connexion with those adjacent, forms a vast 

 subterranean reservoir, which on being penetrated (as often happens) by a 

 coal-shaft, overwhelms the miner with torrents of water. To exclude this dis- 

 charge, cylindrical shafts of masonry are constructed, of sufficient strength 

 to resist the pressure of a column of water, equal in height to that of the 

 highest level at which the water stands in any part of the chain of caverns 

 intersected. These shafts of masonry are carried to as high a level as the 

 water will rise ; and that is frequently to the surface, so as to form an over- 

 flowing well. Since water always carries with it a quantity of air, the explo- 

 sion of which when compressed might be attended with much danger, the 



* This theory may not be considered as applicable to the yellow dolomite of th« north-eastern 

 counties, on account of its fine texture and regular stratification ; yet even there the dolomite is 

 frequently associated with a breccia [vide Wynch. Geol. Trans., vol. iv. p. 6.] ; and perhaps it 

 deserves remark, that, after running parallel to the great chain of mountain limestone, which, with 

 little interrnption, traverses the counties of Durham, York, and Derby, the dolomite suddenly 

 becomes coterminous with that limestone on the south, and does not reappear in other parts of 

 England, except in the vicinity of similar calcareous chains. 



