TINY ROCK-BUILDERS. 



microscope, would suffice to win general admiration. 

 But their value to the geologist in enabling him to 

 determine ancient physical conditions and the rela- 

 tive ages of the earth's strata is a still stronger 

 inducement to study them. As the coral animal 

 can only fulfil its life functions in water where there 

 are no muddy deposits and which is of a rather 

 warm temperature, the occurrence of fossilised corals 

 in those parts of the earth which are now too cold 



FIG. 32. Favosites 

 GatMandica. 



FIG. 33. Tubes and tabulae of 

 F. Gothlandica. 



for their growth indicates vast climatal changes. 

 As to the evidence they present of subsidences and 

 upheavals of ancient ocean beds, to which Mr. 

 Darwin drew special attention in his account of the 

 formation of reefs and atolls, considerable discussion 

 is now going on, and many differences of opinion 

 amongst authorities exist ; but we must not at this 

 point enter into that question. 



We will first settle upon what we mean by corals, 

 and determine the place to which they are usually 

 assigned in the zoological scale. 



