Features and Growth of Lake Goovgarrie. 121 



--and protect the underlying rocks from erosion. Consequently, such 

 rocks rise as hillocks with a central quartz reef. The uneven 

 ground facilitates slight corrosion by water, and, in the lowest por- 

 tions, deposition of fine silt a few inches thick. Where debris from 

 'different reefs meets, " stone fields," and to a less extent, ** pave- 

 ments," of white quartz result. 



(3) Rock Basins. — True rock basins, by which are meant hollows 

 with " live " rock surrounding them, are believed by the writer to 

 •occur in the lake floor, west of the main portion of the lake. The 

 rocks are either decomposed porphyritic epidiorite or decomposed 

 upturned sediments. These basins occur where the quartz reefs just 

 referred to are so numerous. The basins are usually roughly cir- 

 • cular or elliptical in shape; range in their longer diameters from 

 20 or 30 yards to 12 chains or more; and are so shallow (not 

 l)eing more than a few inches below the rock rim) that it is difficult 

 by the eye alone to determine the difference of level. 



The basins are occupied by fine aqueous silts a few inches thick; 



and after rain, water remains in the hollows and has no outlet, 



■except where, as in some localities, an outlet appears to have been 



cut by water from an originally closed basin; and so one basin may 



connect with another by a shallow water channel. 



The Silt Floors of the Lake. 



Where the lake surface is not rock-floored, it is covered by a 

 detrital deposit consisting usually of fine silt, composed chiefl.y of 

 mud and subordinated of vei-y fine sand. Taking the lake as a 

 whole, by far thcii greater area is covered by silt, but to what depth 

 is not known. This silt is usually of a dark red colour, owing to the 

 contained oxide of iron, although the actual floor of the lake becomes 

 white OAving to the formation of a film of salt. No fossils have 

 been found in the silts. The latter are, however, impregnated with 

 -common salt; and gypsum is abundant in the form of crystals from 

 a quarter of an inch to four or five inches in length. The silts 

 below the surface are practically always moist. Their thickness over 

 most of the lake is not known, but towards the southern end bores 

 showed that from where tlie rock floor ended at the western end of 

 the Causeway Island to the eastern shore at the end of the CauscAvay, 

 a distance of about a mile, the rock bottom sloped gradually to the 

 east, until it was apparently about twelve feet below the lake surface 

 ,at the eastern shore. Further information is desii'able as to other 

 portions of the lake, the portion tested by the bores being narrow 

 compared with areas in the lake farther north. 



