PLATE xvn. 



FIG. I. 

 PLAGIOCLASE-AUGITE ROCK, NEAR GRANITE. 



TOLCAKN, NEAR PENZANCE. 



Magnified 40 diameters. Ordinary light. 



The minerals represented are uralitic and actinolitic hornblende (8), tourmaline 

 (14), secondary magnetite, modified and secondary felspar (10) and apatite (15). 



On the right hand side of the figure is a portion of a large patch of fibrous 

 hornblende, in which the axes of the fibres are arranged with a certain amount of 

 parallelism. This is what is indicated by the term uralitic. The term actinolitic is 

 applied to detached needles of hornblende, which may often be observed in the colourless 

 felspar substance. These are not well seen in the figure. 



Tourmaline is represented by a group of three individuals, one of which is cut 

 exactly and the other two nearly at right angles to the vertical axis. The three 

 individuals evidently radiate from a common centre. They show a beautiful zonal 

 structure due to the alternation of the blue (indicolite) and brown varieties. The 

 individual occurring to the right is penetrated by a colourless needle, giving straight 

 extinction and possessing the refractive power of apatite. The colourless substance 

 splits up under crossed nicols into irregular portions of considerable size, which 

 extinguish simultaneously. This is doubtless felspar, but that it is largely of secondary 

 origin appears to be proved by the fact that it contains needles of actinolitic hornblende. 

 The magnetite in the portion represented appears to be mostly of secondary origin and 

 to be associated with the uralitic hornblende. 



FIG. II. 



ANOTHER VARIETY OF THE SAME ROCK. 



Magnified 25 diameters. 



This figure shows augite (7), uralitic and actinolitic hornblende, brown mica (16), 

 felspar, and ragged plates of titaniferous iron ore (5) partially changed to leucoxene. 

 The augite is a pale pinkish variety. It is changed at the margins and along cracks into 

 uralitic and actinolitic hornblende. 



The brown mica occurs in the form of minute overlapping scales, which cannot 

 be resolved into distinct individuals with the magnifying power employed. It is the 

 mineral which is so highly characteristic of contact alteration by granite, and which 

 occurs both in the altered sediments and in the altered eruptive rocks of Cornwall, 

 the Harz and other localities. The felspar substance occurs as a rule in large irregular 

 plates, without twinning ; sometimes, however, it occurs as minute granular aggregates 

 (felspar-mosaic) . That it is of secondary origin at any rate in its present form appears 

 proved by the occurrence in it of inclusions of actinolitic hornblende and brown mica ; 

 both secondary minerals in this rock. 



The rocks represented in this plate were first described by Mr. ALLPOKT. 

 (Q.J.G.S.,Vol. XXXII., 1876, p. 418). 



