IOD MICROSCOPIC TEXTURE OF SLATE. 



volcanic matter upon the sea-bed near to volcanic centres. Two such 

 masses occur in the middle part of the Atlantic, one obviously derived 

 from the volcanic islands to the west of Northern Africa, and the other 

 derived from the volcanoes of the West Indies and Central America. 

 These are areas occupied by red clay which Mr. Murray attributes to 

 the decomposition of pumice and other volcanic materials, finding 

 among the clay many fragmentary crystals of felspar, augite, and other 

 of the minerals which occur in volcanic rocks. This material has 

 been transported by the winds, and decomposed on the sea-bed often 

 at a depth of between two and three thousand fathoms. These regions 

 of the ocean abound in concentric nodules of black oxide of manganese. l 



Kaolin, or china clay, is actually manufactured from the granite 

 at St. Austell in Cornwall, and near Plympton in Devonshire, by 

 breaking up the decomposed rock, and washing it on an inclined 

 plane, with pits arranged to catch the quartz, tourmaline, mica, and 

 other minerals, until the purified clay obtained from the felspar is 

 deposited in tanks and dried for export. Geological deposits almost 

 identical in character occur in the Bagshot Sands, especially at Poole 

 and Wareham. It may be interesting to remark that the tesselated 

 pavements in general use are produced by compressing dry clay till 

 it becomes solid, though as a rule heat is subsequently applied to 

 increase the hardness. 



Clays when sandy are termed loam ; and when calcareous are marls. 

 Occasionally, as in the Kimmeridge Clay, bituminous beds of con- 

 siderable extent occur, and impart to the clay a combustible quality, 

 enabling it to be burned as fuel, a totally different property from the 

 spontaneous combustion sometimes set up in clay cliffs by decomposi- 

 tion of the iron pyrites. 



Slate. Many of the so-called clay slates differ from modern de- 

 posits of clay in containing a very small amount of kaolin material 

 and an immense amount of mica, in flakes too small to be visible to 

 the naked eye, but which give a sort of silky lustre to the rock when 

 they lie in the plane of fracture, as in some of the black slates near 

 Llanberis. 2 With the mica are numerous needles or small black cry- 

 stals like hairs, probably of hornblende and magnatite. 



The Devonian Slates are of similar character. When the crystals 

 of mica have formed in situ, they often abound about special centres, 

 but, as a rule, the mica is evenly distributed, and appears to have been 

 derived from the disintegration of an older rock, such as the micaceo 



- 



:he 

 ;he 



mo 



1 Volcanic ashes and lapilli occur in great abundance in the red clay of th 

 Pacific, which has its centre about the Low Archipelago, and extends towards th 

 Sandwich Islands. The lapilli are all of the basaltic type, but often become 

 glassy. Many of the palagonites are identical in character with those of Sicily, 

 Iceland, and the Galapagos Islands. The most abundant minerals are plagioclase, 

 augite, magnetite, a little sanadine, and hornblende. Quartz is absent. Many of 

 the lapilli are found to be cemented together with zeolites like christianite. 

 Minute crystals forming radiating masses of the same zeolite occur in prodigious 

 quantities in the red clay so as to form, according to the estimate of the Abbs' 

 Re'nard, about a third of its bulk. These zeolites are evidently formed in sit 

 Dr. Sorby has detected similar specimens in the Gault. 



- H. G. Sorby, Quart. Jour. Geol. Soc., vol. xxxvi. 





