252 The Tripolite Deposits of Lilicur. 



Diatomacese make their first appearance in specific areas 

 of the Upper Chalk, increasing thence during Tertiary times, 

 and gain their widest distribution, as well as greatest abund- 

 ance, at the present day, many of the fossil species being 

 identical with those now living. 



Tripolite occurs in many parts of tiie globe, the most 

 noted deposits being those of Tripoli, on the Mediterranean 

 (whence the name) ; Mount Amiata, in Italy; Franzenbad, in 

 Bohemia; and Richmond, in Virginia. The Richmond bed is 

 described as the most extensive of any yet examined, its 

 thickness being 40 feet. 



In travelling between Creswick and the Loddon an ob- 

 server will not fail to mark the occurrence, over an other- 

 wise level tract, of numerous shallow depressions, varying 

 in area from a few perches to a hundred acres and more — 

 now isolated, and again clustered together in twos and threes 

 or dozens within a square mile. Although more extensively 

 existing on volcanic lands they are not absolutely confined 

 to them, as they are occasionally met with on the clay drifts, 

 which usually in this district fringe the lava sheets, as well 

 as on neighbouring Silurian flats. On closer inspection we 

 find that these depressed areas do not present a uniform 

 appearance, for, although they are all covered with water 

 during the greater part of the year, there are some which 

 support a rankness of reeds and coarse grass, while others 

 are free from vegetable growth, and even the scanty bottom 

 layer of humus appears to be entirely deprived of organic 

 matter. In the latter instance we invariably find on the 

 floor and around the margin of the basin a deposit of bog 

 iron ore (earthy limonite). In the marshy pools this 

 ferruginous deposit is absent, although it is evident that 

 there must be an abundance of hydrous peroxide of iron 

 eliminated by the disintegration of the basalt. We have 

 here a good example of the deoxidising action of humic acids 

 generated by decaying vegetable matter. In the other case 

 this deoxidising process has either never taken place, or, if 

 formerly active, has now ceased. The hydrous ferric oxide 

 is precipitated, and, mingling with particles of clay and sand, 

 produces those earthy ironstone sappings and pebbly con- 

 cretions so common on many parts of the plain. These 

 deposits are still in progress of formation, and in course of 

 time will, no doubt, completely obliterate the corrugations 

 which at present mark the face of the land. 



