120 The Microscope. 



siliceous envelope; and he considers that this is a 

 cellulose coat, which becomes interpenetrated by silex. 

 It will be remembered that the delicate cells can be 

 traced in fossil wood (page 92) , when their walls 

 have been similarly penetrated by silex or by carbo- 

 nate of lime ; and, just as they remain almost inde- 

 structible, so do the tiny diatoms, testifying of their 

 existence in past ages, as they occur fossilized in 

 countless myriads in different parts of the world. 



An extensive stratum of earth, eighteen feet in 

 thickness, underlies the whole city of Richmond, in 

 Virginia. When specimens of this earth are examined 

 with the microscope, they prove to be almost entirely 

 composed of the shells, broken and perfect, of dia- 

 toms, in great variety, and exquisitely beautiful. A 

 similar stratum of so-called " infusorial earth" occurs 

 in Bermuda, and a group of the diatoms from the 

 latter are on the microscope's stage while I write. 

 They are all circular, and named Heliopelta ({ shield 

 of the sun," and round as Norval's shield ! See a 

 specimen of Heliopelta, reader, if you can; and if 

 you will examine it with various magnifying powers 

 and modes of illumination, you will be at once asto- 

 nished and delighted at the perfection of its structure, 

 and the wonderful delicacy of its tracery; and will 

 feel pleased at being in some degree informed as to 

 its nature.* 



* The reader will find all the most beautiful diatoms named and 

 briefly described in L. Lane Clarke's " Objects for the Microscope." 



