LACUSTRINE. 



253 



changes that must have taken place, and the ages that must 

 have elapsed since the gravels and sands were piecemeal 

 worn and washed down from their parent formations.* 



Closely associated with these river- deposits, and some- 

 times indeed undistinguishahle from them, are the Lacus- 

 trine or lake deposits, that occupy so many of the alluvial 

 expanses in our plains and valleys. The tendency of 

 every lake fed by running streams is to become shallower 

 and shallower from the sediment deposited in its basin by 

 these inflowing waters. Every stream protrudes its little 

 delta of silt and sand, fresh-water shell-fish accumulate 

 layers of marl, and aquatic plants contribute their annual 

 quota of growth and decay. By-and-by the shallow lake 

 becomes a stagnant morass, and in process of time, partly 

 by surface plant- growth, and partly by the deepening of the 

 outflowing stream, the morass is converted into meadow- 

 land. A large portion of all the " straths " of Scotland 

 and the "dales" of England are of lacustrine formation; 

 and we have only to watch the cutting of any main drain 



* The following is a section of the Uralla gold-field, as given to us some 

 years ago by Mr W. Cleghorn of that district : 



Red rich soil, . . 5 feet. 



Stiff red clay, . 5 



Mottled clay volcanic ashes . 20 



Basaltic lava, . . 35 



Brown laminated clay, . 5 



Loose sand (decomposed quartz and granite) . 2^ 



Black peaty clay, with numerous leaves and stems, 6g 



Loose sand (decomposed quartz and granite) . 2 



Finely laminated reddish clay, . 1 

 Loose sand (decomposed quartz and granite), with 



numerous crystal-pebbles and a little gold, . 15 



Fine reddish clay, . . . . . 

 Loose sand (decomposed quartz and granite), with 



numerous pebbles the main gold deposit, . 4 



102 



Granite, water- worn surface, with large granitic boulders. 



