CHAPTER XI. 



RECENT FORMATIONS. 



Tlie Mitvio-VM trine Deposits. — Little can be added to the descriptions 

 of these ah-eady given. The argillaceous members of the series consist 

 of pijDeclays, more or less ferruginous brick-earths, and sandy loams. 

 As a rule, they contain very few distinguishable minerals other than 

 kaolinite and quartz, although, by careful elutriation, microscopical 

 crystals of zircon and grains of ilmenite occasionally have been detected. 



The sands consist, as a rule, almost entirely of quartz-sand, in places 

 white, but usually of varying shades of yellow and light-brown, accord- 

 ing to the proportion of ferruginous matter which is adhere ut to the 

 grains of quartz. A few grains of sphene, of ilmenite and of zircon, 

 have in places been detected in them, while some beds contain flakes of 

 mica in more or less abundance, biotite being far more prevalent than 

 muscovite. The alluvial deposits are essentially marine borne silts, 

 derived probably from the erosion of fluviatile alluvium ; and their 

 relative freedom from heavy minerals is in accordance with this. 



The shell-beds which in places occur in them contain many kinds 

 of shells, all of which are found at the present time living in the 

 surrounding shallow sea. 



The Residuary Deposits. — The composition of these varies according 

 to the variety of rock they have been derived from. Aplite, granite, 

 pegmatite, quartz-porphyries, the more acidic granitites, and the gneisses 

 and schists derived from them, give rise to more or less sandy kaolins 

 and pipeclays, varying in colour from white to cream-coloured. The 

 hornblendic granites and gneisses and the allied porphyrites and schists 

 produce ochrey-coloured more or less ferruginous clays, whilst the 

 diabase, diorite, epidorite, amphibolite, and hornblende-schist weather 

 into buff-coloured, red, brown, or chocolate-coloured, ferruginous, more 

 or less siliceous earths. These latter consist of mixtures in various 

 proportions of angular quartz-sand and siliceous grit, derived either 

 directly from the quartz originally present in the rock, or secondarily, 

 frum the decomposition of the feldspars; of kaolinite, arising from 

 the feldspars ; and of limonite, or other hydrated oxides of iron, result- 

 ing from the decomposition, hydration and oxidation of the various 

 ferro-magnesian minerals present in the rock. 



A distinguishing feature of the residuary deposits from the fluvio- 

 marine ones is the relatively great abundance of heavy minerals 

 contained in the former. In the residuary deposits from the acidic 

 rocks the unaltered minerals (besides quartz) present in them are 



