THE SOIL. — ^ITS FORMATION AND COMPOSITION. 85 



usually elevated, yield but indifferent crops. In sheltered 

 positions, however, and when it comes into contact with 

 limestone and other formations, it improves ui character. 

 In Scotland the mica slate formation is largely developed, but 

 in England it occurs only in South Devon and in Anglesea. 

 Several specimens of clays produced by the decomposition of 

 the mica slate of Donegal!, and deposited at Milford and 

 other places on the northern coast, appear well adapted for 

 the manufacture of draining -tiles.* 



1 23.— ^GitAYSiJjj:. The rocks which receive this name 

 are found overlying those last mentioned, and occasionally, 

 where the mica slate is absent, rest directly upon the 

 granite. These rocks are of a slaty character and of con- 

 siderable thickness, and the upper layers, which are found to ^- 

 contain remains of shells and vegetables, are usually termed (^ 

 upper slate to distinguish them from those in which no "organic ) 

 remains" have been discovered, and which are described as / 

 lower slate. The lower slate covers a gi-eat extent of the 

 kingdom, resting upon the flanks of the Mourne range, and 

 forming nearly the whole of Down and Louth, except a small 

 portion north of Dundalk. In the south it forms parts of 

 Wicklow, Tipperary, and Cork. In the south also, the upper 

 clay slate extensively prevails in Cork and Kerry, but m 

 Ulster and Connaught it is only to be met with in Galway 

 and Mayo, and in small patches in Tyrone and Fermanagh. 



1 24. The predominating ingredient in the clay-slate rocks is a 

 compound of silica and alumina in the state in which they exist 

 in porcelain clay (115). In addition to these they contain silica 

 and oxide of iron with traces of lime, magnesia, and alkahes. 

 They also frequently contain a mineral named chlorite^] which 



' Analysis of a subsoil resting on decomposing mica slate at Ballyma- 

 cool, Ck)unty Donegal. 100 parts had the following composition: — 



99-10 

 f This mineral is very abundant in the clay-slate rocks of Dovm. 

 Its name is derived from a Greek word, signifying green. 



