90 LESSONS IN CHEMISTRY. 



The chief development of these rocks is in the County of 

 Antrim, where a flood of volcanic matter seems at one time 

 to have been pom-ed over the beds of chalk, converting it 

 into what is, in the north of Ireland, termed " white lime- 

 stone." The volcanic rocks of Antrim border the north-east 

 coast of that county for many miles, forming the picturesque 

 cliffs which give that portion of the country its pecuHar 

 character, assuming, in the Giant's Causeway, the form of 

 angular pillars, and in the magnificent promontory of Fau'head 

 rising in massive columns to a height of 636 feet above the 

 level of the sea. Detached masses of greenstone may be 

 observed protruding in many parts of the kingdom; thus, 

 Croghan Hill, in the King's County, Carlingford mountain, 

 in Louth, and Urrisberg, in Galway, are examples of this 

 rock. The volcanic rocks are of but limited extent in 

 England, but in Scotland they cover a considerable area. 

 They contain a gi-eat variety of ingredients, are readily 

 acted upon by the weather, and the soils which are pro- 

 duced by their decomposition present great variety both in 

 colour and composition. In Ireland they are usually fer- 

 tile, but their character is greatly influenced by position. 

 When they are flat they favour the accumulation of water, 

 and are often covered with peat and marsh, and in many 

 cases the soils produced by their decay contain so much 

 iron as to prove injurious to the growth of the farmers' 

 crops. The iron of the decomposing particles of rock is 

 washed down by the rain, and frequently accumulates in 

 the subsoil, forming an ochrey layer or pan^ which is some- 

 times so hard as to be with difficulty broken up. Upon 

 soils in which this accumulation has taken place, the crops 

 thrive weU until the roots descend to the pan^ when they 

 gradually fail, the excess of iron acting as a poison to the 

 plants. Greenstone is sometimes composed of feldspar and 

 hornblende (see analyses), and in other cases, of the former 

 and augite, a mineral which like hornblende is rich in lime, 

 so that in this mixture we have most valuable ingredients 

 for the nourishment of plants. Below are given analyses of 

 some of these rocks, and also of a soil resting upon them.* 



* The beds of ochre which of various colours are observed along our 

 north-east coast and also in the interior of Antrim have been produced 

 by the decay of volcanic rocks. The following is the composition of 

 Basalt and also of a specimen of red ochre from the cuttings of the 



