486 A. Harlier — Porphyritic Quartz in Igneous Rocks. 



tlie most basic of all rock-forming silicates, has been shown to exist 

 in certain highly acid lavas, but it is known from two or three 

 localities only. The case we have chosen for discussion is the exact 

 opposite of this, viz. the occasional presence of free silica in rocks 

 of thoroughly basic composition. 



Olivine-basalts containing porphyritic quartz have been described 

 by the American petrologists from California, Arizona, New Mexico, 

 and Colorado, and similar cases have been noticed by others in 

 places as widel}' separated as Arran, Madagascar, and the Korea. 

 The typical manner of occurrence of the quartz is in small, more or 

 less rounded grains, coated with a thin layer or shell of a greenish 

 colour. This shell is ascribed to corrosion of the quartz-grain by 

 the molten magma in which it was enveloped ; it consists for the 

 most part of minute crystals or granules of augite, sometimes of 

 hornblende probably pseudomorphous after augite. Except for this 

 shell, the quartz-grains closely resemble the corroded porphyritic 

 quartz of many quartz-por{)hyries. 



There is, however, another group of basic and sub-basic rocks 

 which enclose porphyritic quartz much more commonly than the 

 basalts do, viz. the peculiar group of the lamprophyres (rainettes, 

 kersantites, etc.). The little grains here have the same character as 

 in the preceding case, and are surrounded by a similar augite-shell. 

 They are known in the lamprophyre dykes of the Harz, Spessart, 

 Dresden, and other places, and are beautifully exhibited in the North 

 of England, e.g. in the dykes which cut the Lower Palasozoic inliers 

 of Ed en side and Teesdale. 



The explanation adopted by most writers is that the enclosed 

 grains of quartz represent fragments mechanically caught up by the 

 magma in its passage through solid rocks during the process of 

 intrusion. Such foreign fragments are undoubtedly met with in 

 these as in other intrusive rocks, but to suppose that more than a 

 small proportion of the recorded cases is to be explained in this way 

 raises obvious difficulties. On this hypothesis we should expect 

 inti'usive rocks other than lamprophyres in the same districts to 

 carry as frequently similar fragments : we should look to find in the 

 neighbourhood some rock from which the grains of quartz might be 

 derived ; and we should expect the grains to occur with a local 

 distribution in the dykes which enclose them. The theory fails in 

 these respects, and a closer examination of the grains themselves 

 proves conclusively that they are original constituents of the rocks 

 in which they are found. In the first place they never have the 

 shape of fragments. Whenever the corrosion has not entirely 

 obliterated the ori<^inal form, the outlines of the hexagonal pyramid 

 are clearly discernible. Nextly, the grains are never composite, but 

 always optically uniform throughout their extent. Thirdly, they 

 contain no inclusions except rare crystals of zircon, apatite, or some 

 other mineral of early separation, and glass-cavities, presenting thus 

 a marked contrast to the quartz of granite or gneiss and to the great 

 bulk of the grains in sandstones and grits. To these points of 

 evidence, collectively very strong, others might be added. Thus, in 



