Hyland — On some Spherulitic Rocks from Co. Down. 425 



composition of the magma, which the separation of minerals must 

 produce, and to the rise in temperature which must suddenly 

 ensue upon the transition from the amorphous to the crystallised 

 condition. 1 



The orthoclase is frequently twinned according to the Carlsbad 

 law. The normal orthoclase gives an extinction upon the M- 

 surface of 5 J - 7°. The intergrowth of orthoclase and plagioclase, 

 known as micro-perthite, is sometimes apparent. Sections of such 

 crystals show a number of narrow and irregular strips and patches, 

 arranged with their longest axes parallel to the vertical axis of the 

 crystal. These jjatches extinguish at a different angle to the main 

 mass of the crystal, and appear to be albite. 



In appearance the orthoclase often reminds the observer of the 

 sanidine of trachytic rocks. Its alteration is accompanied by the 

 development of that scaly, granular matter which is generally 

 referred to kaolin. But some of these scales may be recognized 

 as a white micaceous mineral, possessing high double refraction. 

 A certain amount of free silica is set free by this process, and is to 

 be found again as quartz, minute grains of which are seen to be 

 embedded in such altered orthoclases. The decomposition pro- 

 ceeds along the cleavage planes. 



In one of the slides there occurs, lying in the micro-perthite, a 

 colourless mineral, which shows traces of a hexagonal form. It 

 remains almost isotropic under crossed nicols. In its occurrence 

 and general appearance it reminds one of trydimite. Unfortu- 

 nately the mineral is very infrequent, having been only observed 

 three times in the rock : hence, its identity with tridymite cannot 

 be satisfactorily established. The fact, however, that Lagorio has 

 observed the occurrence of this mineral as an alteration product 

 of an " acid " orthoclase, confirms my conclusions upon the point. 

 The rock in which Lagorio made this discovery is the "black 

 Porphyry " of Wallikallio, a locality situated in the East Baltic 

 provinces. The felspars he describes are altered into quartz 



1 Lagorio, I. c, pp. 462, 4G3. See also Kopp : Lieb. Annalen, 1855, 93 125. 

 Scacchi, Palmieri, and Guarini observed tbis rise of temperature upon crystallisation 

 at tbe eruption of Vesuvius in 1855 ; tbe lavas upon which tbis was noticed were tbose 

 in tbe Fosso della Vetrana, (Roth. " Der Vesuv und die Umgegend von Neapel." Berlin 

 1857, 293, 304.) 



2K2 



