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albite-type. The twin axis in this case is normal to the brachypinacoid (010) 

 which plane is also the face of composition. The larger felspars are frequently 

 composed of a number of lamellae due to repeated twinning on the albite plan, 

 and it is this feature which produces the well-known striation on the plane of 

 easiest cleavage (001). Sections at right angles to (010) always give sym- 

 metrical extinctions on opposite sides of the trace of the twin-plane, which in 

 this case is also the composition face. It will subsequently be shown that the 

 observation of a number of such extinctions gives some information as to the 

 particular species of felspar under examination. Next to the albite type of 

 twinning, that known as the pericline type is the most common. In this case 

 the twin-axis is the b axis of the crystal, and the effect of repeated twinning 

 is to produce a striation on the clinopinacoid. It has been shown by YOM 

 RATH that this striation makes different angles with the edge P/M (001 : 010) 

 in the different species of plagioclase. Not seldom the albite and pericline 

 types co-exist in one and the same crystal. 



In addition to the albite and pericline types of twinning, one frequently 

 observes a third type corresponding with the well-known Carlsbad type of the 

 monoclinic felspars. In this case the vertical axis of the crystal is the twin- axis, 

 and the brachypinacoid forms the face of composition. Sometimes, as in the 

 large porphyritic crystals of the doterite (diabase-porphyrite) of EYCOTTHILL one 

 sees that the main individual is composed of two halves due to twinning on the 

 Carlsbad plan, and that each half is again composed of smaller lamellae due to 

 simultaneous twinning on the albite plan. As, in twinning on the Carlsbad 

 and albite plans, the face of composition is usually in both cases the brachy- 

 pinacoid, it is not at first sight easy to recognise in sections whether one or 

 both types of twinning are present. On testing the extinctions, however, it 

 will always be found that if the albite type only be represented, then the odd 

 lamellae all extinguish in one position, and the even lamellae in another position. 

 In other words there are only two positions of extinction, as it were, for the 

 entire section. If the section illustrates simultaneous twinning on the Carlsbad 

 and albite types, it will be found in general that the above rule does not hold. 

 It will be necessary to place the section in more than two positions in order to 

 extinguish all the lamellae. This simultaneous twinning is frequently found in 

 the porphyritic crystals of the rocks now under consideration. In addition to 

 the albite, pericline and Carlsbad types of twinning, there are others which are, 

 however, too complicated to be considered in the present work. 



The porphyritic crystals often show a zonal structure due to inclusions, or to 

 a variation in the optical characters of successive zones. In the former case 

 it is frequently observed that the distance between contiguous zones is greater 

 in the direction of the length than in that of the breadth of the crystal- section ; 

 a fact which testifies to the unequal rate of growth in different directions. 

 Sometimes the inclusions of the groundmass are very irregular in their form 

 and distribution, and so numerous that the crystal appears completely honey- 

 combed by them. This feature is, however, more characteristic of the rocks 

 of intermediate composition the andesites and porphyrites than of those 

 now under consideration. Where the zonal structure is due to a variation in 



