Calcite in Guernsey. 



75 



The rocks of Guernsey are all of the eruptive kind and are 

 mostly crystalline in their structure. They are chiefly granitic, 

 though perhaps it would be difficult to find true granite here. 

 The bulk of the island is composed of Syenites and Gneiss. 

 The Syenites are of many varieties, some being porphyritic. 

 The Gneiss has many peculiarities and is worthy of separate 

 study. 



As you all know we have besides these, a variety of 

 altered eruptive rocks such as Serpentine, Mica Schist, Horn- 

 blende Schist, Chlorite, etc., but in all these no calcite appears 

 as a necessary mineral. We therefore must admit that calcite 

 does not belong to the primary rocks found in Guernsey. 



But we have also derived rocks — that is rocks which have 

 had their rise in the destruction by natural agencies of the 

 primary rocks and it is in these that we first find carbonate of 

 lime. 



In order to account for the presence of lime we must name 

 the rock minerals to be found in the primary rocks. They are 

 1st. Quartz. 2nd. Mica (of two kinds mostly). 3rd. Felspar 

 (of four kinds mostly). 4th. Hornblende. These four minerals 

 and their varieties form the mass of rocks of the island. 



Of these, Quartz contains no Lime, while some varieties of 

 Mica and Felspar do ; and Hornblende is chiefly composed of 

 salts of lime. 



As a large proportion of our blue rocks are hornblendic it 

 follows that lime must be plentiful, and if we look carefully 

 for it in the derived rocks we shall find it. 



By the action of the atmosphere termed " weathering," 

 the rocks are gradually broken up and decomposed. The lime 

 is first carbonised in situ, in which form it is insoluable in 

 water. The continued action of carbonic acid gas and the 

 presence of water produces a soluble bicarbonate, in which 

 form the lime is separated from the rock debris. The water 

 charged with the bicarbonate of lime runs down the crevices 



