12S MR. ,T. MORRISON OV [vol. lxxiv, 



Porphyritic orthoclase and quartz are found in association in all 

 the rocks, from the acid end to the basic. In some they are large 

 and abundant, in others small and sparsely scattered, but (while 

 always conspicuous) show varying degrees of magmatic corrosion. 

 Occurring throughout the mass of an intrusion right up to the 

 bounding walls parallel to which they are usually orientated, their 

 formation is clearly intratelluric. Relative abundance depends 

 neither on distance from the granite nor on the size of the intru- 

 sive mass, near dykes being sometimes less rich than others more 

 remote, and the maximum occurring in the narrowest of the series. 

 Some other factor, therefore, must be sought for the explanation. 



The presence of quartz-grains in basic rocks has frequently been 

 recorded, various explanations having been offered to account for 

 this unusual association. J. P. Iddings refers the corroded quartz- 

 grains in certain basalts to the catalytic action of heated water- 

 vapour during an earlier consolidation of the magma, the vapour 

 weakening the affinity between the bases and the silica and per- 

 mitting them to separate out into more basic silicates and quartz. 

 The consolidation due to pressure is unstable, and, when relief 

 comes, yields to the potential liquidity of the magma. To this 

 period is assigned the partial resorption of the quartz, which is only 

 checked bv the second and final consolidation of the magma. The 

 quartz is thus regarded as native to the basic magma. 



Dr. A. Harker 1 has suggested that the quartz-grains found in 

 certain lamprophyres originated in a magma of acid composition, 

 and sank into an underlying more basic magma, undergoing cor- 

 rosion, not necessarily in the original basin, but probably at a later 

 period when the rocks were intruded into their present positions. 

 This is a simple explanation and, moreover, agrees with recorded 

 observation in the field. 



In the set of intrusions now described felspar and quartz are 

 found in rocks of a somewhat basic type. Their density being 

 greater than that of the liquid magma from which they usually 

 separate, sinking is a normal consequence, provided that the magma 

 retains a sufficient degree of fluidity, and both minerals will ulti- 

 mately be found at a level considerably lower than that of origin. 

 Conditions favourable to the development of this process would arise 

 during the cooling of a magma already differentiated into an upper 

 granitic, and a lower more basic, portion. Similar conditions might 

 prevail if differentiation were deferred to a period subsequent to 

 the intrusion of the granite : refusion of the consolidated magma 

 would follow a relief of pressure, local or otherwise ; and, if we 

 postulate a state of equilibrium between magma and bounding 

 wall as maintained for a sufficient time to allow gravitational 

 differentiation to take place, cooling of the upper layers would 

 initiate the process. A rise of isotherms alone might even be 

 sufficient to liquefy the matrix, and permit existing phenocrysts 

 to settle into the more liquid layers beneath. 



The result of such a process may be briefly considered. By 



1 Geo!. Mag. dec. 3, vol. ix (1892) p. 487. 



