LESSONS IN GEOLOGY. 



Wbilo tli" colony was busy in tho labour of ita life, the clear 

 water of tho Hoa became suddenly turliiil with mud, tho deposit 

 quickly ontombod tint rrinonK aii<l (.. (!;iy tin- Hrodford clay 

 preserves many of thorn in itn, others broken off and lying 



iitally ombeiMc'l in tlm clay. 



ombrask, which is the topmost membor of tho lower 

 is a looso rubbly limeatono ; in some places, however, it 

 becomes solid, and is then quarried for building, as at Malmes- 

 luiry, yet its general character is brashy and disintegrated ; 

 dwinx' to the fact that it it* particularly favourable to the pro- 

 (luctii'ii of good corn-growing soil when it comes to the surface, 

 it has been named " cornbrash." It is by no means a thick 

 deposit, large areas not being more than HIS inches thick , 



is it very fossiliferous, but it passes downwards into an 

 -.ir^illaoeous limestone 



The Forest Marble, which abounds in marine fossils. The 

 eandstone slabs of this bed are frequently ripple-marked, and 



i caused at the same time, and apparently under the 

 i instances. Crystals of sulphate of lime are also common. 

 Many of the ammonites have on each sido of the opening of the 

 month a long projection, as Ammonitet Jaton (Fir 



The Cored rag is the uppermost member of the middle oolite. 



AH its name imports, it is generally a loose, rubbly limestone, 



often almost entirely composed of aggregations of madrepores. 



j Tho two most common indeed, the prevailing corals are the- 



! Theco$milia annularii (Fig. 115) and the Thamnattraa, (Pig. 



II'''). With these corals are found, in great quantities, an 



oyster, the Ottrea gregarea (Fig. 117). One of the Jura lime- 



stones of this period, from its numerous fossils of the Ifcrincea, 



has been called the Nerinean limestone. Fig. 118 is the 



Nerincea Ooodhallii, which is found on the top of the coral rag. 



Besting on the 



THE UPPBR OOLITE. 



ral ratr is the Kimmerida 



iy, the lowest 



IDEAL FLORA AND FAUNA OF THE OOLITE. 

 1, Pterodactyle ; 2, Kaidacarpum; 3, Williamsonia ; 4, Pinites; 5, Mantellia. 



mixed with the broken debris of shells, pieces of wood, etc., 

 plainly indicating that once there tho waters of the sea were met 

 by the land. This stratum owes its name to the fact that in 

 Whichwood Forest, in Oxfordshire, it yields a marble. 



THE MIDDLE OOLITE. 



The lowest member of the middle oolite is the Kclloway rock, 

 an arenaceous limestone usually classed with the Oxford clay ; 

 for in the south-west of England, especially near Kelloway, in 

 Wiltshire, tho clay encloses lenticular masses of the rock, which 

 are very fossiliferous. But as it pas?e? north its dimen- 

 sions increase, until at Scarborough it becomes thirty feet 

 thick. 



The Oxford clay is a vast deposit, its greatest depth being 

 estimated at 700 feet. It is destitute of corals, but ammonites 

 and belemnites are found in any quantities. The plesiosaurus 

 and ichthyosaurus also appear, and a species of crocodile, the 

 Steneosaurus, not unlike those which now bosk upon the mud- 

 banks of the Ganges. 



The fossil wood so frequently found in the clay appears some- 

 times silicified and sometimes converted into an imperfect jet ; 

 it is remarkable that these two distinct conditions should have 



representative of the upper oolite. It consists of beds of blue- 

 slaty or greyish and yellow clay, containing selenite (sulphate of 

 lime). Near Kimmeridge the beds become so highly bituminous 

 that they produce fuel, which is burnt under the name of Kim 

 coal. 



The Ostrea deltoidea (Fig. 119), the Gryphaea virgula (Fig. 

 120), and the Card turn striatulum (Fig. 121), are three charac- 

 teristic fossils of this deposit. 



PORTLAND STONE AND SAND. 



To the Kimmeridge clay succeeds, in the south of England, 

 a bed of sand upon which lie calcareous rocks of various con- 

 sistencies ; from those in the islands of Purbeck and Portland 

 some of our best building stone is quarried. 



In Wiltshire and Dorsetshire many of the beds contain layers 

 of septaria, like the flints of the chalk formation. The Solen- 

 hofen beds in Bavaria belong to the upper oolite, from which the 

 lithographic stone is procured. The beautifully fine grain of 

 this limestone renders it perfect for the preservation of fossils. 

 In 1862 a skeleton of a bird, the Arch&opteryx macritra, was 

 discovered ; some of the feathers even being exhibited. Several 

 species of flying lizards, Pterodactylis (Fi<j. 122), were first found 



