Prismatic Structure in Sandstone, fyc. 249 



to, rendered it necessary to abandon this train of investigation 

 before any satisfactory or decided results were obtained. Thus 

 the subject slept, without being forgotten, till it was revived 

 by the circumstance which I shall now proceed to commu- 

 nicate. 



In taking down a blast furnace at the Old Park Iron-works, 

 near Shiffnall, which had been sixteen or eighteen years in 

 constant work, the hearthstone was found to be broken, and, 

 at the same time, to present a remarkable reticulated surface. 

 On examining the fragments after removal, it was found that 

 they were split into polygonal compartments, and some of them 

 were therefore preserved. One of these was subjected to my 

 examination by the Duke of Northumberland ; in whose pos- 

 session it now is, for the satisfaction of those who may wish to 

 refer to the fact and its authorities. 



The hearthstone, it is well known, lies at the bottom of the 

 blast furnace, so as always to be covered with melted iron at a 

 high heat, and to thisjieat it was unintermittingly exposed during 

 the whole period of years above-mentioned. The kind of 

 stone usually selected for this purpose is a fine-grained white 

 sandstone, containing a very small proportion of argillaceous 

 earth. It is either found, or imagined, to be the least liable to 

 fuse or crack in this high degree of heat. I have not been able 

 to procure natural specimens of the rock from which the hearth 

 in question was made; and it is now impossible to ascertain its 

 exact nature. It can only be concluded that, according to 

 general practice, the hearth in question was made of this 

 variety of sandstone. 



The thickness of the fragment which I examined was about 

 ten inches, which is understood to be that of the hearth. It 

 is everywhere divided into prisms, which, in some places, 

 reach through the whole thickness, but, in others, only pene- 

 trate to a certain depth ; being gradually blended beneath with 

 the solid mass. These prisms are sometimes tolerably regular ; 

 at others, their sides undulate, but so as still to fit each other, 

 without intervals, wherever they approximate. Their diameter 

 varies from one inch to the half of that, and they contain, 

 generally, five, six, or seven sides. They are not absolutely in 

 contact above, but they gradually approximate below ; appear- 



