222 



SCIENTIFIC NEWS. 



[Sept. 7, iE 



Upper Oolitic 



Middle Oolitic 



itic J 



Lower Oolitic 



Purbeck Beds. 



Portland Beds. 



Kimeridge Clays. 



Coralline Beds. 



Oxford Clay and Kelloway Rock. 



Great Oolite Series. 



Inferior Oolite. 

 The inferior Oolite is extensively developed in the Cots- 

 wold Hills, especially near Cheltenham, where it is 

 about 236 ft. thick, but thins out in the direction of Bath, 

 where it is represented by about 25 ft. Near the base 

 of the inferior Oolite at Cheltenham a remarkable series of 

 beds known as the Pea Grit occur (fig. 5,) The name 

 appears to be suggested from the resemblance of the 

 granules to peas, which they do in size and shape. Fig. 

 6 is a micro-photograph of a section of this rock from 

 Cleeve Hill, near Cheltenham, and illustrates the struc- 

 ture and mode of formation of Oolitic strata. The rock 

 is seen to be made up of oval and round granules, the 

 largest of them measuring a little over one-eighth of an 

 inch in diameter, and the smallest - oi4. In each of the 

 granules there is a nucleus around which layers of car- 

 bonate of lime have formed. This is best seen in the 

 large type of granule, which is known as Pisolite, to dis- 

 tinguish it from the smaller or Oolitic grains. In the case 

 of the Pisolite granule in fig. 6 the nucleus is that of a joint 

 of a Pentacrinite, and is shown in section. These crinoids 

 were allied to the Encrinites to which we referred when 

 describing the Carboniferous Limestone, but in the case of 





Fig. 8. — Micro-Photograph of a Section of Forest 

 Marble. ( x 22 diams.) 



the Pentacrinus the joints are five-sided. These creatures 

 were numerous in the Jurassic Sea, and have contri- 

 buted to the strata in the same way, though less exten- 

 sively, that the Encrinites did to the Carboniferous 

 Limestone. The smaller granules in the Oolite have a 

 variety of objects, as nuclei ; sometimes a portion of a 

 spine of an Echinus, a fragment of shell, a foraminifera 

 or grain of sand. 



The Inferior Oolite terminates at the horizon of the 

 Fuller's Earth, a material well known for its use in com- 

 merce. The maximum thickness of the formation is 

 about 400 ft., which includes clays, marls, and nodules 

 of earthy limestone. At Bath, however, it is not more 

 than 150 ft., and continues to decrease in thickness until 

 on reaching the Valley of the Windrush it dies out, so 

 that in the country round Burford the inferior Oolite and 

 great Oolite series are in contact. 



The great Oolite series are thus divided by Mr. H. B. 

 Woodward, in his " Geology of England and Wales : — 

 3. Cornbrash. 

 2. Forest Marble and Bradford Clay. 



1. Great Oolite ! T P per zone " - . 



( Lower zone, Stonesfield slate. 



The name Stonesfield Slate was taken from the village 

 of Stonesfield. The formation appears to be that of an 

 estuarine deposit, for among the fossils which it contains 

 are those of plants, consisting of Algae, Ferns, and Coni- 

 fers. The conditions under which the Stonesfield Slate 

 formed extended over an extensive area in East Glou- 

 cestershire, but is not represented around Bath nearer 

 than at Stroud. Some of the beds yield slabs which are 

 worked for roofing purposes, and hence the term slate, 

 as applied to this rock. 



Above the Stonesfield Slate comes the Bath or Great 

 Oolite, which is extensively quarried on the hills around 

 Bath and mined at Box. Visitors to Bath should en- 

 deavour to visit one of these mines, for they will see a 

 sight which they will never forget. At Bath the Great 

 Oolite is 150 feet thick, and consists of limestone made 

 up of shell debris, tough calcareous rocks mixed with 

 clay and fine oolitic limestones, the latter being the 

 typical " Bath Stone." 



Of the Bath Stone it is very difficult to get a thin- 

 section suitable for examination under a microscope, 

 owing to the oolitic granules being so loosely held 

 together. Directly we begin to grind down a piece of the 

 rock the granules fall out. Fig. 7 is a photograph of a 

 section in which a number of granules remain, but they 

 are not thin enough to show the nuclei. In the figure 

 the spaces between the granules are seen to be filled with 

 infilling calcite, which affords a good example of the pro- 

 cess by which limestone has become compact. The white 

 oval spaces have been made by the falling out of granules. 



At the close of the Great Oolite period there followed 

 conditions which were not favourable to the formation of 

 limestones, and a deposit of clay took place known as 

 the Bradford Clay, the name being taken from Bradford, 

 in Wiltshire, where the clay is best developed. 



Then there was another change of conditions, and 

 limestone-forming organisms again appeared, and their 

 calcareous remains collecting on the sea floor gave rise 

 to a shelly limestone known as the forest marble. Fig. 

 8 is a micro-photograph of this rock, which shows it to 

 be chiefly made up of fragments of shells and some 

 oolitic granules ; but in the photograph the places occu- 

 pied by the granules are represented by circular holes, 

 the granules having dropped out during the grinding 

 down of the section. 



On the other members of the Jurassic S3'stem we must 

 not enter, but the mode of formation and structure 

 differs little from those already referred to. 



The life of the Oolitic period resembled that of the 

 Lias, but we find some fresh genera and species. Corals 

 were numerous ; in the inferior Oolite of the Cotswold 

 Hills there are three fossil coral reefs. Oolitic limestone 

 is at the present day formed in the coral seas, and it is 

 therefore possible, even likely, that the carbonate of lime 

 which formed around the nuclei of oolitic granules may 

 have been derived from that source. Visitors to the 

 Bath Stone quarries should have no difficulty in obtain- 

 ing good specimens of the coral Halamophyllia radiata, 

 which is most numerous in the upper beds. 



[We regret that by an unfortunate mistake figures 

 2 and 3 on page 197 were transposed.] 



