190 PROCEEDINGS OP THE GEOLOGICAL SOCIETY. [Feb. 20^ 



of the iron employed by him in ship -building ; but the rapid destruc- 

 tion of wood necessary to keep up these works caused their speedy 

 abandonment. A large quantity of scoriae from these furnaces was 

 found during the construction of the railway, and even some pigs 

 of iron. Thin veins of hepatic iron-ore occur in many places. It is 

 usually coloured scarlet on its under surface, and has been found by 

 experiment to contain more than 70 per cent, of pure metal. 



In the northern part of the island, at a spot called " Mapou," 

 and in one or two other places, is found a white soft earth locally 

 termed "boue blanche," which supplies the cottars with both plaster 

 and whitewash. It appears to be formed of a mass of decomposed 

 madrepores, but possesses more consistence than would be expected 

 from such a material. 



I now venture to put forth a conjecture as to the probable cause 

 of the existence of the fossil-bearing islands, and the masses of cal- 

 careous rock similar to that which forms them on the shores. 



As fossilization could not have taken place in the air, there must 

 have been a subsidence of the land-bearing forests, carrjdng them 

 below the water-line ; and during the period of their submersion a de- 

 position of sand, which has since formed sandstone, took place. This, 

 probably, only reached to a portion of the height of the trees, and 

 that part which remained exposed to the air rotted away. "Whether 

 the perforations are the work of lithophagous or xylophagous mol- 

 lusks I cannot pretend to say ; but I think it probable that the 

 latter were the agents, as I have seen wood in the sea hollowed in 

 a manner much resembling that in which they are riddled. Petri- 

 faction having taken place, upheaval followed. As the calcareous 

 formation is of considerable antiquity, and is found at the extremities 

 of the island and beyond its proper limits, it may have constituted 

 part of an islan.d larger than Mauritius itself, and of greater antiquity. 

 I have ascertained by careful observation and examination, that the 

 present extent and elevation of Mauritius have undergone no per- 

 ceptible change since the first regular surveys, made upwards of a 

 hundred years ago. 



In conclusion, I may observe that, on most parts of the coast, the 

 land rises, immediately before reaching the sea, to a height of from 

 6 to 20 feet. In the lower parts this rise is occasioned by sand 

 -dunes overlying basalt, and in others by basalt overlying sand. 

 This appears to me to indicate that after a considerable deposit of 

 volcanic matter, at a period long anterior to the formation of Mauri- 

 tius as it now is, a second outpouring took place, over the spots 

 where the basalt is on the surface, and the fluid mass was partially 

 arrested in its downward course by the sand-hills on the shore. The 

 looseness of the heaps of scoriae, which form the elevations mentioned, 

 appears to favour this hypothesis. 



