Clifton College Scientific Society. 51 



rocks were formed which are now filled with ore ; as the two 

 principal hteniatite beds consist mainly of ore, we cannot suppose 

 these open spaces to have been formed when the sand was originally 

 deposited. In the Pennant grit at Portishead it is common to 

 find cavities, formed by the decay of plants buried when the strata 

 were deposited, now filled with oxide of iron. The cavity and the 

 iron, however, distinctly show the form of the original plant. 

 This explanation is not open to us, because there are here no 

 impressions of fossils, and it is impossible to conceive of such 

 a quantity of vegetable matter having been buried at regular 

 intervals on an open shore. 



It is therefore probable that the material which originally filled 

 these cavities was limestone, for we have already seen that lime- 

 stone can be dissolved by water containing carbonic acid, and all 

 sandstone rocks are easily permeable by water. The difficulty is, 

 that we should naturally expect to find traces of the limestone 

 preserved here and there ; but we find nothing of the kind, as far 

 as I know. This difficulty is lessened, if we consider the length 

 of time at our disposal for the complete removal of the limestone, 

 amounting, perhaps, to millions of years. In illustration of the 

 power of water to permeate sandstone rocks and remove carbonate 

 of lime, I exhibit a specimen of sandstone from the upper lime- 

 stone shales, just below the millstone grit, in which crystals of 

 carbonate of lime were formed by segregation. These have subse- 

 quently been entirely removed, leaving only the casts of the 

 double pyramids of calcite. Assuming, then, that the material 

 removed was limestone, the next question is, whether the cavities 

 were all formed first, or whether the two operations of removal 

 and deposition went on simultaneously. 



The existence of stalactites proves that the cavities were formed 

 before the stalactites, but proves nothing about the date of the 

 lining of the walls. 



Some of the cavities in the haematite are divided into an upper 

 and lower chamber, by a floor of haematite, which has stalagmites 

 on its upper, and stalactites hanging from its lower surface, while 

 the central part is compact limonite or red haematite. 



If the cavities in the sandstone had existed before the introduc- 

 tion of any iron, it is impossible to explain the formation of these 

 floors ; for the deposit must have been made on the sides, and 

 stalactites would have hung down the whole length of the cavity. 

 In such cases it appears that the limestone was cut through the 

 middle, probably by a joint through which the water trickled and 

 deposited the compact oxide in a flat sheet ; the work of removal, 

 however, proceeded at a more rapid rate than that of deposition, 

 and thus cavities were left above and below, which afterwards 



