120 G. IF. Tyyrell— Alkaline Igneous Roclcs, Wed Scotlaml. 



Campignien or Early I^eolitliic Beacli. This proved warping, how- 

 ever, has a magnitude of only an inch or two in the mile, and is 

 moreover in the wrong direction to serve as an explanation of the 

 submergence on the shores of Loughs Bannus and Avehy. The 

 smallest tilt that would cover the observed facts should amount to 

 about 50 feet to the mile in the case of Bannus and 20 feet to the 

 mile in the case of Avehy. If such an enormous movement as this 

 has affected the district in general it ought to show itself on the sea- 

 coast, unless indeed the shore-line of the forest period now lies so 

 far beneath the sea that the tilting nowhere brings it above the 

 surface. It is clear that an explanation based on tilting would strain 

 our conceptions of post-glacial warping to the utmost. 



There remains the explanation adopted by certain Swedish geologists 

 to account for the same phenomenon in their own country. This is 

 that the lakes during the growth of the forests had no overflow and 

 sank beneath the level of their outlets. They ascribe this to the 

 relatively dry climatic conditions which they suppose facilitated 

 the growth of forest instead of bog. It is impossible to discuss here 

 the merits of the climatic theory of the peat-bog succession, but it 

 seems reasonable to assume that, if the Swedish cases are due to 

 a period of dry climate, the Irish cases are to be explained in the 

 same way. 



Personally I do not feel at all certain that the mere existence of 

 dense forests over the catchment area of these small lakes might not 

 reduce the amount of water brought in by the tributary streams to 

 such an extent as to lower the level of the lake, or even dry it up 

 altogether except during exceptional floods. If, however, future 

 advances in the study of the economy of forests should prove this idea 

 to be untenable, there would seem to be but small chance of escape from 

 the hypothesis of a very considerable change in the climatic conditions 

 of the remons in which these submerged forests are found. 



VII. — Thk Late PALiEOzoic Alkalink Ignkous Rocks of the West 



OF Scotland. 



By G. W. Tyrrell, A.R.C.Sc, F.G.S., Assistant to the Professor of Geology, 

 Glasgow University. 



(Concluded from the February Number, p. 80.) 

 2. EssExiTE. (a) Carskeoch type. — A small boss of essexite, of rather 

 basic type, pierces the agglomerate of the Carskeoch vent, near Patua, 

 Ayrshire. Macroscopically it is light grej", compact, and fine-grained. 

 In thin section it shows a plexus of fluxionally-arranged plagioclase 

 laths of the composition Ab^ An^, the interstices being filled with 

 subhedral augite of a nearly colourless variety, and fresh olivine. 

 Here and there minute angular interspaces are occupied with turbid 

 isotropic matter, the form and arrangement of the particles of which 

 suggest the former presence of nepheline. A few broad plates of 

 pale augite and crystals of olivine interrupt the general trachytoid 

 fabric. Ilmenite and biotite occur sparsely, and a little orthoelase 

 may be detected on the margins of the plagioclases, extending 

 irregularly into the interspaces. This rock has a distinct individuality, 



