66 



SIENITIC AND EARTHY GRANITE. 



ite to sienite in the same rock, in the granite of Charnwood Foresto 

 The same change may be noticed also in the granite of the Malvern 

 Hills. That able and accurate observer Dr. MacCulloch maintains 

 the identity of granite and sienite, from their frequent passage into 

 each other in the same rocks in Scotland. When the hornblende 

 becomes abundant, and is closely intermixed with felspar, it forms a 

 dark finely granular rock, which has been denominated trap or green- 

 stone : it nearly resembles basalt. In the Charnwood Forest hills, 

 and at Shap in Westmoreland, well defined granite may be seen 

 passing into a dark coloured trap-rock nearly compact. I have even 

 broken off hand specimens, in which one part was granite and the 

 other trap, and the passage from one to the other might be distinctly 

 observed. 



The crystallized earthy minerals which occur most frequently in 

 granite, are schorl or tourmaline, and pinite, a mineral nearly allied 

 to mica, — the emerald, corindon, axinite, and topaz, are also found 

 occasionally in granite. Sometimes, the tourmaline is so abundantly 

 ^disseminated, as to form a constituent part of the rock. 



Common granite, or massive granite, contains few beds of any oth- 

 er rock, nor is it rich in metallic ores. Tin ore, however, occurs 

 chiefly in granite, either in veins accompanying quartz, or dissemina- 

 ted through the rock at a distance from the veins. Ores of other 

 metals, as copper, iron, wolfram, bismuth, and silver, are also found 

 occasionally in granite. 



Granite supplies durable materials for architecture, but it varies 

 much in hardness, and care is required in its selection. I was told, 

 when in Cornwall, that granite, got from a considerable depth in the 

 quarry, is so soft when it is first raised, that it can be easily sawed 

 into blocks, but it soon acquires great hardness by exposure to the 

 air. In the mountains of Auvergne, the granite is extremely soft, 

 and the felspar appears earthy ; this is probably the original state of 

 the stone. I believe it is the soft earthy granite from this district, 

 which supplies the kaolin used in the porcelain manufacture at Sev- 

 res. Mons. Brongniart, who obligingly accompanied me through the 

 works, showed me a specimen of their best kaolin : it contained crys- 

 tals of pinite. I had recently arrived from Auvergne, and I thought 

 I recognised its locality. 



Granite is regarded as the foundation rock on whicli all other rock 

 formations rest, and has hence been called the most ancient forma- 

 tion ; but if the age of a rock is to be dated from the period in which 

 it became consolidated, the inference respecting its relative antiquity 

 would not be conclusive. According to the Huttonian theory, gran- 

 ite is made of the melted crust of a former world, and the fusion may 

 have taken place after this ancient crust was covered with the upper 

 rocks ; but, admitting that it has been fused under pressure, the mat- 

 ter that now constitutes granite must have existed in some mode or 

 other, and have served as the foundation for the rocks that are upon 



