426 PHYSICAL GEOGRAPHY. 



structure of the valve itself is very beautiful and characteristic, but we 

 shall be able here to consider such structure in general terms only, leav- 

 ing its more particular description until we note the peculiarities of a few 

 typical forms, which will be described in detail, and illustrated by figures 

 showing their outline and sculpture, as revealed by means of the micro- 

 scope. The minute sculpture of the diatom valve is commonly spoken 

 of as its "markings," and in all cases they are generally of a similar 

 character on both valves. This, however, is not always the case, as we 

 find in some genera which pass most, if not all, of their lives attached 

 to objects like plants, sticks, stones, and the like, submerged in the water 

 (fresh, brackish, or salt) which the diatomacenc inhabit. In such cases, 

 as they arc dissimilar, and can be distinguished, it has become customary 

 to call that valve which is next to the object upon which it grows the 

 lower valve, and the opposite one the upper, — and if, as must often be the 

 case when the diatom is fixed to the under side of a plant leaf as it floats 

 upon the surface of the water, the position of the valves is reversed. In 

 all cases the valves arc convex at the edges, although they may be some- 

 what flattened near the middle, so that when the edges arc in contact 

 they enclose the cavity already spoken of, and which may be very shallow 

 in proportion to its width, perfectly spherical, or several times deeper 

 than wide. 



The great point of beauty in the diatomaccx, and what has attracted 

 to them the attention of so many unscientific possessors of microscopes, 

 is the symmetry and forms of these same sculpturings or markings found 

 upon the siliceous envelope ; and these vary in delicacy from comparatively 

 coarse reticulations to such extremely minute dots that no microscope 

 has as yet been constructed which will show us what is their true char- 

 acter, so that, as they lie side by side in rows, the siliceous cell-wall 

 appears to be marked with extremely faint lines. But this is not all, for 

 there are diatoms upon which even such faint lines have not been seen; 

 but we know that they must be fashioned after the same manner as their 

 brothers, and must, therefore, possess markings of yet greater fineness, 

 and which time, it is to be hoped, with the aid of improved means of 

 research, will reveal to the inquiring eye of the future observer. The 

 great delicacy of some of the markings found upon diatom cell-walls, and 

 the consequent difficulty of seeing them, has led to great rivalry in 



