THE SOIL. — ITS FORMATION AND COMPOSITION. 89 



magnesia, is termed magnesian limestone. In England these 

 rocks are of great importance, and largely developed, but 

 in this country they possess but little agricultural interest 

 Magnesian limestone is found at Cnltra, near Holywood, in 

 the County of Down, on the shore of Belfast Lough, and 

 varies in colour from a light yellow to brown. It also 

 occurs at Howth, near Dublin, and in three or four other 

 places in Ireland. 



1 30. Though, as we have seen, the limestone formation may 

 be regarded as forming the floor upon which the soil in a very 

 large portion of the island rests, yet it is found that in the 

 central plain the soils, even where placed upon a bed of this 

 formation, possess characters which render it evident that 

 they cannot have been produced by the decay of limestone 

 rocks. Thus, analysis discovers that they are signally defi- 

 cient in lime, and experience shows that their productiveness 

 is materially increased by its application. "In those localities 

 where the clayey diluvial gi-avel is interposed between the 

 surface and the rock, the soil is usually wet and clayey ; so 

 much so as to become impervious to water. Hence has arisen 

 the tendency to the growth of bog moss (Sphagnum Fcdustre) 

 and other aquatic plants, which have gi*adually produced the 

 bogs that occur so abundantly among the Eskers or gravel 

 hills of our central districts. In those parts of the great lime- 

 stone district in which the gravel deposits have not extended, 

 the soil is rich and is capable of producing any kind of agi'i- 

 cultural crop; but in these fertile plains less exertion has 

 been displayed than in other parts of the country where the 

 soil is of inferior quality ; but where, owing to the industry of 

 the people, the quantity and quality of the crop per acre, is 

 superior to that produced on the rich calcareous loams." — 

 (GriffiiJis.) 



131. Xrap or VOLCANIC ROCKS^ The next group of rocks ^c 

 to which I would direct your attention, as affording soils by ) 

 their decomposition, are those which cover a large portion off 

 the northern part of the kingdom, and are kno wn b j^_JJi» ^ 



_na©fia^^^^ba^jtjgreenstonej^ a^^ These rocks 



insider 



are usually coiisidere3^ to have been converted into then- 

 present state by the action of fire, to be in fact composed of ^ 

 matter ejected from volcanoes ; occasionally we observe them 

 forced up in nan*ow ledges between rocks of an entirely 

 different charactei*. These are usually termed whin dykes. 



