Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/563

1869.] granite of the Caldew valley is nearly N.E. and S.W., and therefore very nearly agrees with the strike of the Skiddaw Slates in this region. The syenitic porphyry of Carrock Fell, on the other hand, appears to extend itself in a direction from E. to W., this being very nearly the strike of the bedded traps of the green slate series by which it is surrounded. Secondly, the mineral character of the intrusive rock appears to vary with the character of the rocks which surround it. Thus, high up in the valley of the Caldew we find a coarsely crystalline granite lying in the heart of the Skiddaw Slates, the latter being highly metamorphosed for a considerable distance on both sides of the plutonic rock. As we approach the point where the Skiddaw Slates are overlain by the green slates and porphyries, the granite becomes much finer in grain and less highly quartziferous. Finally, as seen in Carrock Fell, where it is surrounded by the traps of the green slate series, it has entirely lost its granitic character, and is now simply a felstone-porphyry. It contains neither quartz nor mica, and is composed of a base of reddish felspar, with crystals of felspar and specks of hornblende, closely resembling, in fact, the syenitic porphyry of Buttermere.

In investigating the age of these igneous masses, an important element of the inquiry is found in the discovery which I have recently made (Geol. Mag. vol. vi. Nos. 3 & 4), that the green slates and porphyries are superimposed unconformably upon the Skiddaw Slates, an opposite opinion having been hitherto entertained. The Skiddaw Slates, therefore, were subjected to elevation and denudation before the formation of the green slates and porphyries had been initiated. Bearing this in mind, it is, in the first place, noticeable with regard to the three igneous masses in question, that they occupy a definite and similar position in the stratified series of the Lake-district. They are all found to be in relation, on the one hand, with the Skiddaw Slates, a vast group of purely sedimentary origin, and, on the other hand, with the green slates and porphyries, an equally vast series of volcanic ashes and lavas. The second fact to be noticed is that it is only the inferior sedimentary formation that is metamorphosed, to any perceptible extent, by contact with these intrusive masses. Of course, from the nature of the upper group, any alteration which might be present would be much less conspicuous than in the case of the Skiddaw Slates; but I have not been able to satisfy myself that there occurs any. The Skiddaw Slates, however, are invariably metamorphosed in the neighbourhood of the intrusive rock, this metamorphism affecting in different localities a greater or less thickness of the beds. In the vicinity of Buttermere the alteration in the Skiddaw Slates is comparatively slight in amount, and unaltered slates are found not many yards distant from the intrusive rock. At the foot of Ennerdale, on the other hand, the slates, in contact with the same rock, are altered throughout an enormous thickness, the metamorphism being plainly visible at a