4 2 4 PHYSICAL GEOGRAPHY. 



arc elaborated, constructed, and built up. What appears to be the ex- 

 terior membrane of a diatom is siliceous ; that is to say, it is composed 

 of the substance known commonly as silex, but called by the chemist 

 silica. This is the same material as that which, crystallized, we find in 

 rock crystal or quartz, and which, variously colored, constitutes flint, 

 agate, jasper, amethyst, and various other minerals often used for orna- 

 mental purposes. Of course, this portion of the diatom is very hard, 

 and on this account what we may with considerable propriety call the 

 skeletons of dead individuals are used for the purpose of polishing 

 metals, and similar substances, under the name of tripoli, — although it 

 by no means follows that all tripoli is made up of dead diatom skeletons. 

 The typical form of a mature and perfect individual diatom is con- 

 structed after the following manner : The outer wall, consisting, as has 

 just been said, of silica, is formed of two portions so fashioned that when 

 these are united, as they are during the life of the organism, they form 

 a little box, within which is enclosed the softer parts which go to make 

 up what have been called the "cell contents," under the supposition that 

 the diatomaceoe were "unicellular" organisms. Now that the old theory 

 of cell organization has been very materially modified, this appellation 

 had better be discarded, as it has been used (and in fact was constructed) 

 for the purpose of describing a condition, of the existence of which 

 there are considerable doubts. The whole diatomacca^ous individual has 

 been called the frustule, and under this designation we will speak of it 

 here. Within the siliceous wall some observers have thought that they 

 have detected a delicate membrane surrounding the rest of the contents, 

 but it is doubtful if such a limiting membrane really exists ; and, in fact, 

 it is much more probable that the general mass of the contents is made 

 up of protoplasmic substance, consisting, like such protoplasm usually, 

 of a more or less semi-fluid material, without any differentiation of its 

 parts from the centre to the exterior. Enclosed within the mass of this 

 clear, colorless protoplasm are seen irregularly formed masses of a sub- 

 stance of a greater consistence, and of a peculiar light yellowish-brown 

 tint: this is known as the endochrotne. Sometimes the endochromc 

 occurs in the form of two masses, but often there are many such masses. 

 In either case, as a common thing, the particles of endochromc are so 

 disposed as to lie near to the two portions of the siliceous wall which 



