EXERCISES ON THE LIAS POEMATION. 425 



certain period, threw down deposits of clay, or mud, ceased 

 to do so after a time, either from a change in its currents, or 

 from the destruction or removal of the land which had 

 supplied the substance previously precipitated ; under altered 

 conditions the same waters might deposit beds of sand, 

 or afford conditions favourable for the growth of zoophytes, 

 and thus produce the argillaceous, arenaceous, and calcareous 

 deposits observable in different portions of this group. 



EXEECISES 



ON THE LIAS FOEMATION. 



1. Explain the meaning of the term lias. 



2. Mention the chief authors and collections. 



3. Name the distinctive features of the formation. 



4. Mention the principal discoverers and collectors. 



5. Describe the characteristic shells and fossils. 



6. State the division of the beds. 



7. Trace the geographical distribution of these deposits in 

 this country and on the Continent. 



8. Name the counties in which they occur in England. 



9. Point out any particular phenomena indicated by their 

 organic remains. 



10. Name the chief localities in which fossils have been 

 discovered. 



11. Mention the most recent discoveries effected in these 

 deposits. 



12. State any facts of general and instructive interest, and 

 pursue the prescribed method of question and answer. 



