114 T HE MICROSCOPE. 



the remains of sponges, for it is not uncommon to 

 find the external forms and markings characteristic 

 of their organisms preserved, whilst thin sections of 

 flint show a spongeous texture in the interior. Fora- 

 miniferal shells, and bodies termed Xanthidia^ with 

 their long spinous projections (the sporangia of Des- 

 midiaceae), are often found embedded in flint. The 

 siliceous spicules of sponges are also found in jaspers 

 and agates. The pretty little round concretions of the 

 Oolitic formation, so conspicuous in Bath stone, have 

 been formed concentrically round a nucleus which is 

 often a foraminiferal shell. The green sands which 

 occur in various deposits from the Silurian to the 

 Tertiary period, and which, when occurring beneath 

 the chalk, are known as the Greensand formation, 

 have been shown by the microscope to consist of the 

 siliceous casts, coloured by silicate of iron, of fora- 

 miniferal shells, or those of minute Mollusca. I must 

 not forget to mention the discovery in late years, by 

 Dr. Carpenter, of the nature of the serpentine limestone 

 in the Laurentian formations of Canada. This deposit 

 consists of a regular series of stratified rocks, and 

 underlies the equivalents, not merely of the Silurian, 

 but also of the Upper and Lower Cambrian systems 

 of this country. We are told that these rocks spread 

 over an area of 200,000 square miles, and that they 

 are composed of a species of foraminiferal shell called 

 Eozoon Canadense. "The geological position of this 

 fossil, indicating the vast remoteness in time of its exist- 

 ence as a living organism, is scarcely more remarkable 

 than its zoological relations ; for at what (so far as we 

 at present know) was the dawn of animal life upon 

 our globe, it affords evidence of a most extraordinary 

 development of that rhizopod type of animal life, 

 which now presents itself only in forms of compara- 

 tive insignificance, a development which enabled it to 

 separate carbonate of lime from the ocean-waters, in 



