62 GEOLOGICAL FORMATIONS NEAR EMBLETON 



remains are concerned. One of the very few rocks which 

 there is any reason to regard as having been formed about 

 this time is the great intrusive sheet of dolerite which is so 

 well known under the name of the Whin Sill. Regarding this 

 rock there seems to remain even yet some imperfections of 

 knowledge, and as that is the case, a few words may be 

 given here with the object of clearing up the more obscure, 

 or less known, points in connection with it. 



As just now remarked, the Whin Sill is an eruptive rock 

 allied in composition to basalt, which has been formed within 

 the Earth's crust, and has, therefore, not been poured out on 

 the surface in the form of a lava-flow. One important conse- 

 quence of this feature in its history is that the fluid rocks cooled 

 much more slowly than would have been the case had the rock 

 been at the surface. Besides that factor there is another one 

 of importance, which is connected with the fact that it 

 consolidated not only at a slow rate, but that it did so under 

 great superincumbent pressure. As a consequence of these 

 conditions, the rock has become entirely crystalline, or holo- 

 crystalline, and its component minerals are arranged, with 

 respect to each other, in a somewhat unusual manner. The 

 essential minerals of which it consists are ferro-magnesian 

 silicates which belong to a group of rock-forming minerals 

 known as the Pyroxenes. Occurring in about equal proportion 

 to these are numerous small lath-shaped crystals of one of 

 the lime-soda felspars, Labradorite. There is always present, 

 too, a small, but variable, percentage of the ferrous titanate 

 Ilmenite ; and other minerals, such as Apatite, may also occur. 

 But the two essential^ are the Pyroxene and the Labradorite. 

 The order of consolidation of these can readily be made out by 

 the study of thin sections under the microscope, especially 

 when polarised light is used. The iron ores (and some other 

 constituents) consolidated first. Then the lath-shaped crj'stals 

 of Labradorite crystallised out of the magma, which consisted, 

 at this stage, almost entirely of what was destined to become 

 Pyroxene. Finally this last-named mineral gradually passed 

 into the crystalline state, and in doing so crystallised out in 

 large patches, each of which enclosed some of the minerals 

 which had passed into their finished state at an earlier stage. 

 The structure arising in this way is known as Ophitic. It is 



