A. Harker — Lamprophyres of North of England. 203 



and less acid portions, which, if gravity be the controlling agent, 

 must form upper and lower strata within the reservoir. There can 

 be little doubt that such a hypothesis provides a vera causa for 

 many of the phenomena. Now if we compare a more acid with 

 a more basic type in the normal series of igneous rocks, we find 

 certain chemical relations to hold with a high degree of generality. 

 As the silica-percentage diminishes, the proportion of iron-oxides 

 increases (especially at the most basic end of the series), the 

 magnesia increases steadily, the lime increases and then falls off 

 again, the total alkalies diminish, and the proportion of potash 

 to soda also in general diminishes. All systematic treatment of 

 ordinary igneous rocks which is in any degree ' natural ' (as opposed 

 to Linnsean) is tacitly based upon these general laws. 



We are regarding the lamprophyres as basic modifications of 

 various plutonic rocks, and it is easy to see that tested by the above 

 laws they are abnormal, the exceptional characters being found in 

 the behaviour of the alkalies. This appears on comparing the 

 analysis of a lamprophyre with that of the plutonic rock with which 

 it is certainly or presumably connected. Take, for instance, the 

 biotite-granite of Durbach in the Black Forest, and the remarkable 

 lamprophyre (the durbachite of Sauer^) which forms a marginal 

 modification of it. The analyses give — 



Silica. Soda. Potash. 



Granite 67-70 3-22 5-78 



Lamprophyre ... 51-05 1-85 7-24 



showing that with a heavy falling off in silica the total of the two 

 alkalies remains closely the same as in the normal rock, while the 

 ratio of potash to soda, instead of diminishing, rises from 1-79 in 

 the granite to 3-91 in the lamprophyre. Again, the quartz-bearing 

 augite-syenite or akerite of Ramnas passes at its margin into a 

 lamprophyric rock, and the figures are as follows : ■ — 



Silica. Soda. Potash. 



Akerite 58-48 5-52 3-06 



Lamprophyre ... 46-40 4-81 3-84 



Here, as before, the total alkalies remain nearly the same, and the 

 ratio of potash to soda increases from 0-56 to 080. The augite- 

 minette of the Plauen'schen Grunde may faii'ly be compared with 

 the syenite which it cuts through, and the results stand thus — 



Silica. Soda. Potash. 



Syenite 69-83 2-44 6-57 



Lamprophyre ... 50-81 1-01 7-01 



the total alkalies only falling from 9-01 to 8-02, and the ratio of 

 potash to soda rising from 2-69 to 6-94. 



Judged by these examples, the lamprophyres would seem to be 

 special basic modifications of their parent-rocks in which, with a 

 greatly diminished percentage of silica, the total alkalies show little 

 change, while potash becomes more abundant at the expense of 

 soda. We have selected, however, cases of lamprophyres in the 



^ Mitth. Grossherz. Badea Landes, vol. ii. p. 258. 

 * Erogger, Syenitpegmatitgdnge, p. 49. 



