§ 



GJ* Outlines of the Mimralopj and Geologij 



Tl 



e veins which traverse it are small, antl not parallel to each 

 other; they are principally Quartz aud Epidote^ and at West- 

 Cambridge it is traversed by veins of Sienite. The Siilphurets 

 of Iron and Copper, the former sometimes magnetick, are the only 

 metals which have been discovered in it. The Sulphuret of Iron 



bed at C 



IV. PORPHYF.Y. 



1. This is a compound rock, having a compact basis, in 

 which are imbedded crystals or grains of other minerals, of co- 

 temporaneous formation. 



varieties of Porphyry, and they derive their 



their bases ; the basis which predom- 



II. Th 



of 





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mates in this vicinity is Petrosilex, and consequently, the Por- 

 phyry is the Petrosileceous Porphyry ; the imbedded minerals 

 Which form the other constituent parts, are quartz and Feldspar. 



e colour of the basis is generally of some shade of red, be- 

 tweeu chocolate and brownish purple red, but it also occurs bluish 

 black and greenish grey ; the colour of the Feldspar is white, oft- 

 en with a tinge of green, grey or black, and its lustre is shining 

 and pearly ; frequently approaching to vitreous, or it is dull. 

 Similar colours are presented by the Quartz; it is dull, and oft- 

 en has a greasy aspect ; and it has a delicate splintery fracture, 

 very similar to that of some varieties of Petrosilex. 



III. The imbedded minerals are in nearly equal 

 the Feldspar predominates in some specimens, and the' Q 



prop or 



r 



othei 



The grains of Feldspar and Quartz vary from small 

 10 muidie siz.d, and in the black varieties they are very niinute ; 

 the Feldspar is generally in rectangular crystalline grains, and 



the Quartz in small rounded nodules. 



