GEOLOGICAL FORMATIONS NEAR EMBLETON 63 



characteristic of dolerites. As a comprehensive definition of 

 what constitutes a dolerite has not yet found its way into text- 

 books, one such may be given here. It will embrace under one 

 name a great variety of rocks to which those who devote much 

 time indoors to the microscopic study of rock specimens are 

 fond of calling by a perplexing variety of names, all but one 

 of which are quite unneeded. The definition is as follows : — 



DoLEiiiTE, an eruptive rock of basic composition, i.e., which 

 contains not more than fifty per cent, of silica, and which 

 consists, essentially, of one or more of the Pyroxenes, usually 

 Augite, with about an equal proportion of a basic felspar, 

 usually the lime-soda felspar Labradorite. The felspar occurs 

 in lath-shaped crystals which have consolidated at an earlier 

 stage than the Pyroxenes, and by patches of which latter 

 mineral the felspar lath-shaped crystals are usuallj' enclosed. 

 Accessory minerals are commonly present, of which the two 

 following are the chief. Some iron ores, usually the ferrous 

 titanate Ilmenite and the ferro-magnesian ortho silicate Olivine. 

 With these may occur small needles of Apatite (phosphate of 

 lime). Dolerite can never occur as a lava, but only as a rock 

 which has consolidated within the Earth's crust. It usually 

 occurs in the form of a sill (or stratiform sheet), and less often 

 as a dyke (or wall-like sheet). Stated in a still more concise 

 form, a dolerite may be defined as a holo-crystalline eruptive 

 rock of basic composition and ophitic structure. A true basalt 

 is never hrlo-crystalline in structure, but exhibits, in all cases, 

 some trace of a glassy, or else a lithoidal, ground mass, 

 which has arisen through the comparatively rapid cooling of 

 the part of the rock which was in a fluid state when the 

 final stage of eruption took place. 



Being an intrusive rock, the Whin Sill has made its way 

 into the enclosing sedimentary rocks in such a manner as to 

 occur on any one of widely separated geological horizons. At 

 Embleton, for example, it occupies a position amongst the 

 Carboniferous Eocks between the Eight Yards, or Underset, 

 Limestone and the Great, or Main, Limestone, and is thus 

 quite a thousand feet higher than it is in Westmoreland, close 

 to the village of Helton, near Appleby. On the other hand, the 

 Whin Sill occurs in Upper Carboniferous Rocks near Brampton 

 in North Cumberland, where its geological position uiust be 



