THE MATERIALS OF STRATA. 49 



deposits formed within the lake will be mainly 

 calcareous. Fresh-water plants like the Chara 



precipitate carbonate of lime upon the stem by 



absorbing the carbonic acid gas from the water, 



that the carbonate of lime is no longer soluble. 



This ensures an accumulation of granular lime- 



te as the plants decay. Such a deposit covers 

 up the remains of fresh water shells, and fre- 

 quently the remains of animals derived from land. 



The lakes in Cumberland and Westmoreland 

 are found to have their beds covered with deposits 

 which consist of volcanic minerals when they lie 



in r< ecu pied by old volcanic rocks; while 



the deposits are ordinary sediments when the 

 lakes are surrounded by rocks formed of such ma- 

 terials. If the lake is sufficiently large, like the 

 Lake of Geneva, the sediments may be completely 



irely deposited, and pass from the 



condition of tnd boulders where 



the Rhone coming from the Valais enters the lake, 



to the finest sediment where its Clear waters leave- 

 it, southward of Geneva. Hence a lake may con- 

 tain an epit ill known water-formed rocks 



— pebble-beds, sands, clays, limestones —as well 



as layers of plant remains consolidated into 

 lignite. 



Examples of such fresh-water growth of sedi 

 ments alternating with lignite, has been already 

 referred to in the layers known as Coal Measures. 

 In the lacustrine deposits which are so important 

 in the northern part of the Isle of Wight, fresh- 

 water limestones are familiarly seen at Headon 

 Hill and Bembridge, which were formed in fresh- 

 water lakes, and give no evidence of sediments 

 being mixed with the calcareous matter. Other 

 fresh-water limestones alternating with terrestrial 

 d 



