LESSONS IN GEOLOGY. 



121 



from tho fact that only eighteen per cent, of the fossils of the 



one arc to be found In tho other, a fact which indicates a vast 



lapse of time between their respective depositions. The Gaolt 



la a dark-grey, blue, or brown clay, BO stiff that it is used for 



lirii-kiimkiiig, M in the case at Cambridge and Folkestone. The 



fossils of the Gault are admirably preserved, and so definite as 



to enable tho deposit to be traced in many parts of Europe. 



The Upper Greenland bears a strong lithological likeness to 



1 ireensand. Beds and concretionary masses occur 



and are called in Surrey firestone*. The greeiiHand is 

 -i;:iil y looked upon as the littoral deposit of the cretaceous sea. 

 hi t)i<> greensand at Cambridge, a bird about the size of a wood- 

 cock has boon found, which is introduced into our sketch of a 

 scene of tho period. 



'.. We now enter the distinctive deposit of the epoch. 



land the chalk deposit reaches a thickness of 1,000 feet. 

 The upper beds contain flints, then follows a great mass of 



bonate of lime have been mechanically suspended in the sea, M 

 we see it in at the foot of the chalk cliffs ? The answer to this 

 is, that the sea never has sufficient chalk in it to two op all the 

 carbonic acid gas in its water. There is always present in the 

 sea-water such a quantity of gas that all the chalk brought 

 down by a river, or eroded by the waves washing a chalk-bound 

 coast, might easily be rendered soluble; and unless we hare 

 some very convincing evidence that thing* in the cretaoeotu 

 epoch were not as they are now, we have no right to imagine 

 that tho constitution of the atmosphere and of the sea was 

 at all different in those distant times. Moreover, there art 

 at present agencies at work which are quite capable, time being 

 granted and Nature never is restricted to time to complete 

 her works to construct masses of chalk of any size. We 

 find all the shoH-fiith, and many other of tho inhabitants of the 

 ocean world, endowed with a power of secreting the lime held in 

 solution in the sea- water, in order to build for themselves protec- 



1. Equisetum. 



IDEAL FLORA AND FAUNA OF UPPER CRETACBOUS PERIOD. 



2. Ichthyosaurus. 3. Plesiosaurus. 4. Bird from Greendsand. 5. Pterodactyls. 6. Pinus. 7. Cycadec. 8. Turtles. 



chalk without flints, and under this is chalk marl, which 

 id chalk with an admixture of argillaceous matter. 



Chalk is pure carbonate of lime, usually white, though it may 

 be found grey and even red. The area in which it appears 

 stretches from Ireland to the Crimea, a distance of 1,140 miles, 

 and extends in breadth from Sweden to Bordeaux, 840 miles ; 

 and throughout this vast deposit, it retains its homogeneous 

 composition carbonate of lime and nothing else. Even this 

 fact alone would make us look at the chalk suspiciously as a 

 sedimentary deposit ; and when wo consider tho circumstances 

 under which carbonate of limo is held in solution in water, and 

 how it may be precipitated, our suspicions are confirmed. Wo 

 must seek for other causes to account for the existence of chalk 

 than the ordinary deposition from water which held it mechani- 

 cally suspended. 



Water lias no solvent power on carbonnto of lima, unless it 

 bo charged with carbonic acid gas. The water of tho sea, as all 

 water exposed to the atmosphere, contains this gas, and hence 

 it is capable of holding a little chalk in solution. This chalk 

 could only be deposited if the gas were driven out of the water, 

 which might be done either by raising the water to a tempera- 

 hire approaching its boiling point, or by evaporating tho water, 

 awny : to neither of these conditions could the water of tho sea 

 ever have been submitted. It may be asked, could not the car- 



tive coverings. In the Southern seas the coral polype has built up 

 enormous reefs of limestone, some of which edge the Australian 

 continent for more than 1,000 miles ! Islands innumerable 

 stud the Pacific Ocean which almost all owe their existence to 

 the indefatigable little workers, who for ages untold have taken 

 from the very waves which seemed to threaten them with a 

 furious destruction the limy material wherewith to build their 

 habitations. Each generation added to the work of their 

 fathers, thus raising from the watery depths a wall of rook. 

 The aggregate efforts of tho nation of the coral polypes has 

 produced a mass of limestone well deserving of a place in the 

 geological system were it but embedded in the crust of the 

 earth. But although from the coral fossils found in the chalk 

 wo may conclude that those deposits partly owe their existence 

 to the coral polype, yet other workers shared in the construc- 

 tion. By carefully pulverising a pioco of chalk, and allowing 

 tho grains to settle after throwing them into water, upon 

 i examining them under the microscope it will be discovered that 

 j most of the grains are either perfect shells or parts of shells. 

 i They belong to the family of tha Bhizopods or radiate animals, 

 j for they have long filaments issuing- from their shells which 

 I have tho appearance of root* ; hence their name, radix (a root). 

 I They are also known as Fora mini/era, because the shells, to 

 1 allow the passage of the roots, ara perforated with holes 



