A. Sarker — Porphyritic Quartz in Igneous Rocks. 487 



the lampropbyres of the North of England, and apparently of some 

 other districts, the quartz-grains are accompanied by various acid 

 felspars. These never have the form of fragments, but of perfect 

 crystals rounded hy corrosion, and they are to be matched, not in any 

 rocks broken through by the dykes, but in moi'e acid rocks contem- 

 poraneous and cognate w^ith the lampropbyres. 



Diller and Iddings, clearly recognizing in the case of the American 

 basalts the primary nature of the quartz, have supposed that this 

 mineral separated out at an early stage from a magma having the 

 composition of the rock in which the grains occur. Petrologists in 

 general vfill, I think, be slow to admit the possibility of this. The 

 last-named geologist^ has pointed out that crystallization in an 

 igneous magma may be modified by many conditions — the presence 

 of water, variations of temperature and pressure, etc. — the effects of 

 which we cannot always foresee ; but it may fairly be objected that, 

 if all these factors were of much importance, we should never find 

 two specimens of igneous rocks alike. The exceptional nature of 

 the phenomenon to be explained seems to preclude such general 

 considerations. 



A quite different and perhaps more plausible explanation of the 

 presence of quartz-grains in basic rocks is suggested by an hypothesis 

 which has recently attained some prominence in geological specu- 

 lation. I refer to the conception of a large body of molten rock- 

 material becoming separated by gravity into strata of diiferent 

 densities, the upper and lighter layers being the more acid, the 

 lower and heavier more basic in composition. Postulating such a 

 divided magma, we obtain a clue to the cognate origin of acid and 

 basic rocks in many areas of igneous intrusion. There is another 

 consideration essential to our argument. It seems to be established 

 that crystals of quartz and felspar are not only denser than the 

 magma which gives birth to them, but also denser than magmas 

 of much more basic composition. If, then, we suppose quartz to 

 crystallize out in the upper acid portion of a stratified magma-basin 

 at an early stage, when the magma is still quite fluid, we see that 

 the crystals must sink into the lower basic layers of the heterogeneous 

 mass. 



Our theory is, then, that the quartz was crystallized out, not in 

 the basic rock in which it now occurs, but in a magma of acid 

 composition which once overlay the basic ; so that the quartz-grains 

 scattered through our lamprophyre dykes had precisely the same 

 origin as the quartz-crystals in the accompanying dykes of quartz- 

 porphyry. The grains indeed differ from the crystals only in their 

 more advanced stage of corrosion, due to the more basic nature of 

 the enveloping magma. This process of corrosion, however, need 

 not have taken place in the original magma-basin, and it is more 

 probably assigned to a later time, when the gi'eat pressure was 

 relieved, and the rocks were injected into their present position. 



These ideas, suggested by a study of our north-country rocks, will 

 probably be found to apply to the lampropbyres of other districts, 

 1 J. P. Iddings, Amer. Journ. Scl (3) vol. xxxvi. p. 208 (1888). 



