298 NATURAL SCIENCE. 



June. 



district. This dyke is ten metres thick. The centre is composed of 

 porphyritic crystals of orthoclase, and a fine-grained matrix of biotite, 

 orthoclase, plagioclase and quartz. Pyrite, magnetite and apatite 

 occur as accessory constituents. As the border is approached, 

 magnetite, pyrite and biotite increase, plagioclase takes the place of 

 orthoclase, and the rock becomes finer in grain and darker in colour. 

 Quartz is entirely absent from the marginal rock. The rock of the 

 centre contains 6171 per cent, of silica, and is a porphyritic mica- 

 syenite; that of the margin contains 43'33 per cent, of silica, and is 

 a basic kersantite. A narrow vein runs off from the main dyke at 

 one point. The rock of this vein becomes more and more basic as 

 it is traced away from the junction, and at the termination contains 

 15 per cent, of magnetite. There is no evidence whatever of suc- 

 cessive eruptions in this dyke. The transition from the acid to the 

 basic rock is perfectly gradual. The minerals are concentrated in 

 the order of their formation, and the phenomena are, therefore, 

 capable of explanation by Soret's principle. Seeing that the basic 

 portions of the dyke are vertical, that is, parallel with dyke-walls, it 

 is impossible to explain the differentiation by the local accumulation 

 of the first-formed crystals. Magmatic differentiation appears to be 

 necessitated by the facts of the case. 



Somewhat similar phenomena have been observed by Professor 

 Lawson (3) in the basic dykes of the Rainy Lake region of Canada. 

 The differences are not so strongly pronounced, but they are of the 

 same general character. Thus, in five dykes, the percentage of 

 silica in the rock from the centre was found to be in excess of that 

 in the rock from the margin. In the White-fish Bay dyke the 

 marginal rock contained 47-5 per cent., and the rock from the centre 

 52*47 per cent. Along with the increase in silica there is a decrease 

 in lime, iron, and magnesia, and an increase in alkalies. The texture 

 of the marginal rock is trachytic, that of the intermediate rock ophitic, 

 and that of the centre granitoid. 



Mr. Iddings has described a set of related plutonic and volcanic 

 rocks (2) occurring in the Yellowstone National Park. The sequence 

 in both cases is the same, and the latest rocks are the most acid. 

 Thus, referring only to the intrusive rocks, occurring as dykes and 

 stocks, pyroxene-porphyrites with a holocrystalline ground-mass pass 

 into, and are succeeded by, pyroxene- and hornblende-diorites, these, 

 again, by hornblende-biotite-diorites, and these by porphyritic 

 quartz-biotite-diorites. As the author remarks, the whole series is a 

 single, irregularly interrupted succession of outbursts of magma that 

 gradually changed its composition and character. Eleven analyses 

 are given, showing a gradual increase in silica from 56-28 per cent, to 

 69*24 per cent. 



The writer of the present article, in conjunction with Mr. Dakyns, 

 has recently described an area of plutonic rocks occurring near 

 the head of Loch Lomond (5). The oldest rocks of the complex are 



