146 TOURMALIN I;-IU;aK1\() KOCKS, Ml'. HKEMSKIKK. 



With the exception of a little muscovite, there is no mica 

 present in our sections. It may be said that the mica is 

 generally present in much smaller quantities than in the 

 normal granites, and in some cases it appears to be entirely 

 absent. 



From the above description it will be seen that the tour- 

 maline granites differ from the normal granites in the fol- 

 lowing particulars: — 



(1.) The greater amount of variation exhibited by the 



tourmaline granites, both in the mineralogical 



composition and in the size of the constituent 



grains. 

 (2.) The frequent presence of miarolitic cavities. 

 (3.) The presence of macroscopic bundles of radiating 



tourmaline or of quartz tourmaline nodules. 

 (4.) The absence of microscopic rods of tourmaline in 



the quartz. 

 (5.) The presence of small quantities of muscovite, 



which appears to be absent from the normal 



type. 

 (6.) The lesser abundance of biotite. 

 The tourmaline aplites, as has already been stated, occur 

 in irregular m_asses in tourmaline granites, and as dykes in 

 both the tourmaline granites and the normal granites. In 

 neither case is there any sign of parting at the contact 

 between the aplite and the granite. In the case of the dykes 

 the walls are well defined, but there is no distinct plane of 

 division, the one rock appearing to " grow into " the other. 

 This phenomenon is ascribed by Eosenbusch^ to the fact 

 that the aplite was introduced into the fissures while the 

 granite was still in a highly heated condition. In the case 

 of the masses the contact is even less sharp, and it is some- 

 times impossible to say within two or three inches where the 

 aplite begins and the granite ends. Occasionally the masses 

 of aplite throw out irregular apophyses into the surrounding 

 granite. The whole appearance of the masses gives the * 

 impression that they were formed while both ma2:mas were 

 still liquid, and one may ascribe the irregular shape of some 

 of them to movements in the still molten magma. The 

 aplites are usually much richer in tourmaline than the 

 tourmaline granites. The mineral is, however, confined 

 to the quartz tourmaline nodules, which are often very 

 abundant, and generally are almost perfect spheres. The 

 quartz tourmaline nodules withstand the decomposing 

 action of the atmosphere for a longer time than the sur- 



* H. Rosenbusch. Elemt-nte der Gesteinslehre, p. 215. 



