24 



THE POPULAR EDUCATOE. 



LESSONS IN GEOLOGY. XXIII. 



THE JURASSIC FORMATION- OOLITE. 



To the Oolite is attached a peculiar interest since William 

 Smith, "the father of English Geology," lent his sagacious 

 mind to tabulate its subdivisions first. 



It runs in a band, varying from thirty to ninety miles in 

 breadth, from the Yorkshire coast, a little south of the Tees, to 

 Lyme Regis in Dorsetshire. When we cross this belt, passing 

 from the south-east to the north-west, we find it consists of 

 three series of low hills 

 separated by consider- 

 able valleys. The three 

 hills are the three divi- 

 sions of the system 

 the Upper, Middle, and 

 Lower Oolite ; whilst 

 the clays of the forma- 

 tion are to be found in 

 the valleys, a fact which 

 points to the reason of 

 the existence of the 

 hills and dales. The 

 following is a table of 

 the members of the 

 system : 



f Purbeck beds. 



-j Portland fa tone. 



^Kimmeridge clay. 



Kiddle 

 Oolite. 



Lower I 

 Oolite. 1 



Oxford clay. 

 Kelloway rock. 

 Cornbrasb. and 

 Forest marble. 

 Great oolite. 

 Stonesfield slate. 

 Fuller's earth. 

 Inferior oolite. 



THE LOWER OOLITE. 



Superincumbent upon 

 the Lias in the south 

 and west of England 

 are certain yellow 

 sands : these are the 

 sands of the Inferior 

 Oolite. They evidently 

 bear a close connection 

 with a calcareous free- 

 stone, for they pass 

 into this rock, or give 

 place to it. The sand 

 and the freestone form 

 the inferior oolite, and 

 together have but small 

 thickness. 



The Terebratula, 

 fimbria, Rhynconella 

 spinosa, and the Phola- 

 domya fidicula are its 

 sharacteristic fossils. 

 rhere are thirty-nine 

 species of cephalopoda 

 known in the inferior 



oolite ; but, what is extremely remarkable, only one of the Beletn- 

 nites, the B. giganteus, survived the period ; and this is the more 

 strange, for the great oolite is also a calcareous deposit, and 

 would give us the idea that the same circumstances existed 

 during its deposition as those under which the inferior oolite 

 was formed. The thin layer of fuller's earth, which is only of 

 local occurrence, cannot represent any violent change ; hence it 

 is difficult to account for the extinction of thirty-eight species 

 of cephalopoda. 



Fuller's earth is a thin argillaceous deposit which occurs near 

 Bath, but is wanting 1 in the north of England, and separates the 

 inferior from the great, or Bath oolite. The most plentiful of 

 its fossils is a small oyster, Ostrea acuminata. 



The Stonesfield slate in Oxfordshire is a slightly oolitic, 

 shelly limestone, but aa it passes northwards into Northamp- crust of the great oolite is covered with quite a forest of crinoids. 



tonshire it gives evidence of a more decidedly marine deposit, 

 assuming more of a sandstone character. In Oxfordshire, as 

 at Colleyweston near Peterborough, the Stonesfield slate is rich 

 in organic remains. Several insects have been discovered, 

 and the wing-covers of beetles, beautifully preserved, indi- 

 cating a close proximity to the land. But by far the most 

 celebrated fossils yielded by the Stonesfield slate are the jaws 

 of certain small mammiferous quadrupeds,' which at least indi- 

 cate the existence of three distinct genera, the Amphitherium, 

 Phascolotherium, and Stereoynathus (Fig. 113). The fact that 



the jaw-bone is in one 

 piece indicates that it 

 did not belong to a fish 

 or to a reptile. The 

 number of teeth, their 

 double fangs, and the 

 complicated crowns of 

 the molars, besides tie 

 peculiar structure of 

 the hinge of the jaw, 

 all tend to show that 

 the creature was a 

 mammal. 



It was noticed that 

 the lowest process or 

 projection at the hinge 

 end of the jaw was 

 bent inwards. Now 

 the marsupial quad- 

 rupeds, which are the 

 stepping-stone between 

 creatures who bring 

 forth their young alive 

 and those who propa- 

 gate their species by 

 eggs, have this pecu- 

 liarity of the jaw-bone ; 

 hence the mammals of 

 the Stonesfield slate 

 were pronounced to b& 

 marsupials ; but this 

 cannot be affirmed, for 

 the bend of the process 

 is not great, and there 

 are certain placental 

 mammals whose lower 

 jaw exhibits a certain 

 inflection of the pro- 

 cess. 



The flora of the era 

 was extensive, as may 

 be gathered from tho 

 fact that at Brora, in 

 Sunderlandshire, there 

 is the thickest stratum 

 of coal found in any 

 English secondary 

 rook. It has been mined 

 for a long period, the 

 bed being three and 

 a-half feet thick. 



Tho Great or Bath 

 Oolite, in point of thick- 

 ness and of utility, is the most important of the series. It 

 mainly consists of a stratified calcareous mass, which varies in 

 thickness from 130 to 200 feet. It affords an excellent build- 

 ing stone. St. Paul's Cathedral is built of stone quarried 

 from it, from Burford, in Oxfordshire; near Bath, the stone 

 has a finer grain. 



The great oolite contains so many corals, that some parts of 

 it deserve the name of coralline limestone. The Eunomia 

 radiata is very plentiful, and appears to have grown like the 

 brain coral of our time, probably centuries being required by the 

 polypes to construct the masses, which are found several feet in 

 diameter. 



The crinoids or stone-lilies are also abundant. At Bradford, 

 near Bath, an interesting section is exposed where the upper 



