TIDESWELL DALE QUARRIES. 7 



1 have compared the cracks with those produced in the muddy 

 bottom of a pond. There is, however, an important point of 

 difference. Whereas the cracks produced in the bottom of a pond 

 (resulting as they do from the continued action of the sun's 

 heat during a dry season) are comparatively deep, those in our 

 coral mud are merely surface cracks, the deepest of them being, 

 in those specimens which I have seen, never more than ^th of an 

 inch in depth. From this, and several other considerations, I 

 conclude that these cracks have been produced between the periods 

 of high water. But then, why should they not have been 

 obliterated by the returning tide ? For the same reason that the 

 ripple-marks, rain-pittings, and footprints, so well known in certain 

 sandstones, were not obliterated. To make this clearer, let me 

 briefly summarise the story of these stony hieroglyphics as I 

 interpret it. 



There existed in the region of our quarry, in the old limestone 

 sea, a coral reef, on which, in the tropical climate of the period, 

 myriads of coral polypes lived, reproduced their kind, and died. 

 Existing coral reefs do not, however, consist entirely of the 

 remains of the coral polypes. Large masses of the true coral 

 rock are broken off by the action of the breakers on the outside 

 of the reef, some of which are flung up and accumulate above 

 high water mark. Others are ground down by the action of the 

 waves into a fine calcareous mud. Observations on coral reefs 

 show that a considerable portion of the shores of the lagoons are 

 covered with this mud. Our laminated and cracked specimens 

 are undoubtedly of this nature. The laminations indicate fresh 

 additions of mud brought by the waves, and during one of the 

 intervals between high water the cracks were produced. On the 

 return of the tide a fresh layer of mud was deposited, which filled 

 up the cracks, and protected them from the further action of the 

 waves. The hammer of the quarryman or the geologist, has 

 split the mass of hardened limestone along this plane of slight 

 cohesion, and the cracks and their casts are revealed to the 

 human eye. 



It was during the time that this little page in the world's history 



