I08- THE MICROSCOPE. 



the sounding-lead from the bottom of the Atlantic, at 

 depths of from 1,000 to 2,000 fathoms. We are told 

 that they were probably more abundant during the 

 later geological periods, having been detected by Pro- 

 fessor Ehrenberg in the chalks and marls of Sicily and 

 Greece, and of Oran, in Africa, and also in the 

 diatomaceous deposits of Bermuda, and Richmond 

 (Virginia). 



" It is an admitted rule in geological science, that the 

 past history of the earth is to be interpreted, so far as 

 may be found possible, by the study of the changes 

 which are still going on. Thus when we meet with 

 an extensive stratum of fossilised Diatom acese in what 

 is now dry land, we can entertain no doubt that this 

 siliceous deposit originally accumulated either at the 

 bottom of a fresh-water lake, or beneath the waters of 

 ' the ocean ; just as such deposits are formed at the pre- 

 sent time by the production and death of successive 

 generations of these bodies, whose indestructible casings 

 accumulate in the lapse of ages, so as to form layers 

 whose thickness is only limited by the time during which 

 this process has been in action. In like manner, when 

 we meet with a limestone rock entirely composed of 

 the calcareous shells of Foraminifera, some of them 

 entire, others broken up into minute particles, we 

 interpret the phenomenon by the fact that the dredg- 

 ings obtained from some parts of the ocean-bottom 

 consist almost entirely of existing Foraminifera, in 

 which entire shells, the animals of which may be yet 

 alive, are mingled with the debris of others that have 

 been reduced by the action of the waves to a frag- 

 mentary state. Now in the fine white mud which is 

 brought up from almost every part of the sea-bottom 

 of the Levant, where it forms a stratum that is con- 

 tinually undergoing a slow but steady increase in 

 thickness, the microscopic researches of Professor 



