Geological Excursion to Cheltenham, etc. 525 



" Pea-Grrit," with its first Coral Eeef, at Crickley Hill, was next 

 described. Then the Freestones and Oolite Marl with a second 

 Coral-bed found therein; and, lastly, the Upper Eagstones and third 

 Coral-bed. 



Dr. Wright handed round some of the leading fossils of each of 

 these beds, and gave a detailed description of them. The oolitic 

 or ' roe '-structure of these rocks had long engaged the study of 

 naturalists, and lately he had read a paper to the Cotteswold Club at 

 Bath on the genesis of the Oolites. He said he regarded all Oolitic 

 Limestones as the product of Coral Eeef builders, either in the form of 

 fossil reefs as that specimen in his hand, or as the debris of Coral 

 which had been rubbed down into coral mud, and re-arranged along 

 the coral strand in the form of Oolitic grains, for it was only along 

 the shores of coral islands that Oolitic structure was discovered 

 among modern formations, for reef-building corals can only live and 

 work in water having a temperature of 80° to 82°, and that con- 

 dition is now chiefly found between 30° north and south of the 

 equator. Their operation is for the most part confined within those 

 limits ; while even in those latitudes where the ocean is traversed by 

 cold currents, there zoophytic life is absent. 



Dr. Wright then explained how, in certain areas of the earth's 

 surface, one portion is subsiding whilst another is rising, and obser- 

 vation had proved that the region of the coral sea of our time is a 

 vast area of depression, and that as the life-conditions within that 

 area were most favourable to the development of polyjps, there was 

 a prodigious amount of Eeef-building going on. When they built 

 around a shore, it formed a fringing reef; when that ground became 

 depressed, a barrier reef was formed, which encircled the island ; and 

 when the land became finally submerged, rings of coral or " Atolls," 

 like Whit-Sunday Island, were formed, and alone remained to attest 

 the former existence of terra-firma. He pointed out that when a 

 Eeef breaks up, the rock is reduced to a fine mud or into coralline 

 sand, and that this calcareous paste coats fine particles of sand, and that 

 these particles, having been rolled to and fro by the swell of the ever- 

 restless ocean, such a motion is imparted to the grains that Oolitic 

 limestones are thus formed around the shores of the coral islands of our 

 day. Having thus established the fact that Oolitic limestones are 

 produced under these conditions, he proceeded to apply this to 

 explain the origin of the Oolites now under consideration. Nearly 

 all Oolitic limestones contain Coral Eeefs. There were three super- 

 imposed in the Lower Oolite, one in the Great Oolite, another in the 

 Forest Marble, several in the Coralline Oolite, and one certainly in the 

 Portland Oolite. These Eeefs were the undoubted products of 

 Zoophytes, and when the Eeef, which had grown to perfection and 

 life in it, had ceased, that calcareous mass became disintegrated and 

 rubbed down to mud, and this mud became the paste out of which 

 beds of Oolitic limestones were formed. This theory enabled us to 

 explain the remarkable cross-bedding in many Oolitic quarries, and 

 likewise as coral limestones might have been piled up under sub- 

 aerial conditions along the shores of islands, their great thickness iu 



