Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/564

440 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [June 23, distance of half a mile from the syenitic porphyry. In the Caldew valley the metamorphism is still greater, and affects a much greater thickness of the beds.

In the third place, the intrusive masses show a great similarity in their mineral characters. They are all highly felspathic, and the chief difference between them is in the amount of free quartz which they contain, this being, however, variable in different portions of even the same mass. In the syenite of the Vale of St. John, the amount of quartz is tolerably large; but in the syenitic porphyry of Buttermere quartz is much more sparingly developed; and in the felstone-porphyry of Carrock Fell it appears to be entirely wanting.

Fourthly, the series of the green slates and porphyries, where it comes into relation with these intrusive rocks, is distinguished by the absence or thinness of the breccias and ashes, and by the great development of the bedded traps, the former expanding and the latter diminishing in thickness as we get at a distance from the intrusive focus. Thus, the syenite of the Vale of St. John is succeeded to the south by the great series of bedded traps, which extend to the foot of Thirlmere, the breccias and ashes of Borrowdale and Watendlath having entirely thinned out, though they are of great thickness and are situated only a few miles to the S.W. In like manner the felspathic trap, which usually forms the base of the green slate series, and which is generally of small thickness, appears to expand enormously in thickness to the S.W. of the syenitic porphyry of Ennerdale, as is seen in the valley of the Liza. Lastly, the felstone-porphyry of Carrock Fell occurs in connexion with the well-known series of bedded traps which compose the Caldbeck Fells, and which constitute the inferior portion of the green slates in this region.

Fifthly, in the case of one of these intrusive rocks (the felstone-porphyry of Carrock Fell), it seems tolerably certain that there is a gradual passage from a felstone, through a fine-grained granitic rock, into a genuine granite.

Taking the above-mentioned facts into consideration, it appears to me that two theories only could well be advanced to account for the relations which subsist between these intrusive masses and the stratified rocks which lie around them.

It might be held, in the first place, that these intrusive rocks belong in reality to the Skiddaw Slates, the date of their production coinciding with the period in which the Skiddaw Slates were primarily elevated, anterior to the commencement of the deposition of the series of the green slates. In this case the green slates and porphyries would be subsequently deposited upon a denuded surface formed partially of Skiddaw Slates and partially of intrusive igneous rock. To this view there are various objections, the chief being that the intrusive rock in some cases appears to penetrate the superior formation by which it is almost entirely surrounded, as is the case with the felstone-porphyry of Carrock Fell.

The other theory has been well expressed by Professor Ramsay (Mem. Geol. Survey, vol. iii. p. 235) with regard to some of the in-