208 



the vertical axis. Another special feature of these crystals is the 

 presence of diallagic lamination parallel to the basal plane. This 

 striation is best exhibited in clino-pinacoidal sections, when it is seen 

 to make an angle of about 74 with the vertical axis and, in the case 

 of a simply twinned crystal, to produce a kind of herring bone lineation 

 with reference to the trace of the face of composition (100). 



The coarser varieties of the Whin Sill also frequently contain 

 micro-pegmatite which plays the role of interstitial matter exactly as 

 it does in the more acid gabbros of Carrock Fell. Brown hornblende 

 sometimes occurs as an accessory constituent. It is found intergrown 

 with the augite in such a way that the plane of symmetry is common 

 to the two minerals. 



The general structure of the normal rock may be described as 

 sub-ophitic. The augite which is pale in colour occurs for the most 

 part in granular aggregates rather than as large plates exhibiting 

 uniform extinction over wide areas. Nevertheless, it is frequently 

 penetrated by the lath-shaped felspars in such a way as to show that 

 it is, on the whole, of later date. In other words there is a marked 

 approach to the ophitic character. Compared, however, with the typical 

 ophitic olivine-dolerites of Tertiary and Carboniferous age and with the 

 ophitic non-olivine bearing dolerites (diabases) of Lower Palaeozoic age, 

 the ophitic character is seen to be very imperfectly developed. 



The felspars belong, on the whole, to one generation. The exam- 

 ination of a large series of microscopic slides, however, reveals the 

 occasional presence of felspars which are earlier in date than those 

 of the main mass of the rock. These are never known to occur in such 

 numbers as to give a porphyritic character to the rock. 



The iron-ore is both magnetic and titaniferous. It is, in all proba- 

 bility, an intergrowth of magnetite and ilmenite. 



The rhombic pyroxene occurs by itself and also intergrown with 

 augite. It is not present in all sections, at the same time it is by no means 

 a rare constituent. It shows faint pleochroism of the characteristic type 

 and is generally more or less altered to a green fibrous substance (bastite). 



Brown mica occurs occasionally as an accessory constituent. Chlorite 

 and pyrite are found among the alteration products. 



At its junction with the sedimentary rocks the Whin Sill becomes 

 finely crystalline and even compact. The compact variety shows in 

 thin section a micro-porphyritic texture. Small, more or less lath-shaped 

 sections of felspar lie embedded in a ground-mass composed of minute 

 and evenly distributed specks of iron oxide and extremely small felspar 

 microlites. In the thinnest preparations these microlites are superposed on 

 each other so that it is impossible to speak with certainty as to the 

 presence or absence of interstitial matter. There is the closest possible 

 resemblance between the ground-mass of the compact rock and that of 

 many andesites. The finely crystalline rock which occurs a foot or two from 

 the junction presents a very different type of micro-structure. It appears 

 under the microscope at first glance to be holocrystalline and to consist 



