The Triioolite Deposits of Lilicur. 251 



the nature of similar diatomaceous deposits generally. Con- 

 sidered from a mineralogical standpoint, the Lilicur material 

 agrees with the variety Randanite of the Auvergne, in 

 France — i.e., it is an opal, consisting essentially of hydrated 

 silica, with about 6 per cent, of alumina and lime. As a 

 rock-forming substance it is known by petrologists as 

 tripoUte. Examined under the microscope, it is found to 

 be almost entirely made up of the frustula of diatoms. The 

 more familiar name, " infusorial earth" implying, as it does, 

 a zoogenic rock, was given under a misapprehension of the 

 true nature of the organism of those microscopic bodies. 



DiatomacecB, or hacillaria, form a suborder of the thallo- 

 phyta unicellularia, and may be concisely characterised as 

 single-celled alg^ with silicious valves. They are aquatic 

 plants, thriving alike in fresh, brackish, or salt water ; under 

 the Equator, or in the Polar regions ; on the bed of the 

 ocean, under a covering of 27,000 feet of water, or ice-bound 

 among the glaciers of the highest mountains. The individual 

 diatom is constructed of a germ nucleus, surrounded by a 

 watery plasma, which is contained within a silicious enve- 

 lope. Propagation takes place by partition along the centre 

 of the nucleus and the entire cell. The period during which 

 the diatom remains in an undivided state has been variously 

 estimated at from six to forty hours ; taking twenty-four 

 hours as the average, it will be seen that a single diatom is 

 capable of multiplying to the extent of upwards of a 

 thousand million within the space of a month. 



On the death of the plant the empty frustules fall to the 

 bottom of the basin in which it grew. Here, being mainly 

 composed of silica, and therefore practically unaffected by 

 decomposition, they accumulate, building up layer upon 

 layer, often over considerable areas, and, under favourable 

 conditions, actually filling up the entire space within the 

 basin originally occupied by water. 



The number of species of diatoms known exceeds 1500, 

 and although the forms of some fresh- water genera bear a 

 close resemblance to others of brackish, and even salt-water 

 habitats, it is often possible to judge from the type of diatoms 

 present whether a tripolite is of fluvial, lacustrine^ or sub- 

 marine origin. It can be easily conceived that in a lacustrine 

 deposit, especially if the lake had no regular inlet (as those 

 of Lilicur), the distinct genera are likely to be few in number, 

 and often endemic, while an estuary or submarine bed would 

 generally contain a large variety of both genera and species. 



