220 HOUSE, GARDEN, AND FIELD 



fragrant than on a chalk- down. Our Yorkshire chalk is 

 not so thick nor so soft as the chalk of the southern counties, 

 and the hill-forms differ a little. The prominences are 

 not so evenly rounded, and the gullies not so sinuous. 



What is chalk, and how was it made ? I do not venture 

 to tell that story after Huxley's lecture on a Piece of 

 Chalk, which is accessible to all readers. We know or 

 can easily get to know that chalk is a calcareous paste, 

 formed on the floor of an ancient sea, that it is very similar 

 in microscopic character to the Atlantic ooze, which now 

 overspreads the almost level ocean-floor between Ireland 

 and America ; and that it consists, like the ooze, in great 

 part of the shells of Foraminifera and minute calcareous 

 seaweeds. The chalk is more purely calcareous than 

 the Atlantic ooze, and was probably formed in shallower 

 water. 



How long is it since the chalk was formed ? We have 

 no measure for such intervals of time, and all estimates 

 are misleading. Some have measured the thickness of 

 such a formation as the chalk, guessed at its rate of forma- 

 tion, and then by rule of three figured out the time re- 

 quired for its formation. Except by such methods we 

 have no means of conjecturing when the chalk began to 

 form, nor how long its formation was in progress. It will 

 be more useful to apply a test of antiquity which attempts 

 no arithmetical precision whatever. The chalk is the 

 latest product of what may be called the mediaeval period 

 in geological history. The modern period, and especially 

 the later divisions of the modern period, are marked by 

 the occurrence of animals which are yet living. The fossils 

 belonging to the latest of these divisions include many 

 quadrupeds which still live ; going further back, the 

 quadrupeds disappear, and are replaced by strange forms. 

 The shells change much more slowly than the quadrupeds, 

 but as we trace the life of successive periods and divisions, 

 we find that the shells too change with some regularity. 

 In the later tertiary formations a majority of the shells 



