84 LESSONS IN CHEMISTRY. 



When hornblende is present, the agricultural capabilities of 



the soil will be of a higher character; in addition to the 



potash supplied in abundance by the decomposing feldspar, 



the soil will then contain a large amount of lime and magnesia. 



/ 1 22. — MicaSij^^e. Resting upon the sides of the granite 



f mountains ancTcovering a considerable surface of the country, 



J especially in Ulster, where it constitutes the greater part of 



/ Donegal, Derry, and Tyrone, we find this rock, which derives 



J its name from mica (120) being its chief ingredient. It is 



^Iso met with in the north-east of Antrim, in the glens 



between Cushendall and Ballycastle. It is not found in 



Munster, but in Connaught it is the prevailing rock, and in 



the west of Mayo extends from Broadhaven to the sound 



of AchUl, and it also spreads over a considerable surface in 



Galway. It is met with in a few spots in Wicklow. This 



rock splits into slaty layers which usually exhibit some 



appearance of the shining particles of mica. Its composition 



varies very much, and the soils which rest upon it exhibit 



considerable variety of character. It decomposes very slowly, 



and in general affords but thin and poor soils, which, bemg 



bulk. Its usual colour is pale white, but it occasionally occurs of a 

 flesh-red. It consists of silica, potash, and alumina. The siUca is 

 united as an acid with potash, forming a silicate of potash, (50) and 

 with alumina, forming a silicate of alumina. The last of these com- 

 pounds is the porcelain clay which in England is of so much value. 

 From its composition, feldspar in granite readily decays. The rain 

 charged with carbonic acid unites with the potash, thus forming soluble 

 carbonate of potasli, which is dissolved, while tlie insoluble clay, the 

 cohesion of the rock being destroyed, is carried along and deposited in 

 the valleys, leaving behind on the hills merely a sterile sand and gravel. 

 Feldspar can be scratched by quartz. 



Hornblende, as it occasionally occurs in the granite of the County of 

 Down, (syenitic granite), is of a dark green or black colour and usually 

 crystallized. It does not, like mica, split into layers when heated. 



A specimen of granite from Annalong, County Down, examined 

 in my laboratory had the following composition : — 



100-00 



