62 DR. A. E. DWEEEYHOUSE ON INTRUSIVE EOCKS [Feb. 1909, 



growth is by far the most common felspathie ingredient of the rock. 

 The orthoclase apparently plays the part of host, the inclusions of 

 plagioclase being irregular in shape but usually having their 

 greatest dimensions in the direction of the orthodiagonal of the 

 orthoclase, and being so arranged that their twin lamellae (albite- 

 law) are parallel to the clinopinacoid of the orthoclase. In many 

 instances the plagiolase-inclusions are of sufficient size to allow of 

 their extinction being measured ; and, as the result of a large number 

 of such measurements, using the twin striation as the index-plane, 

 I have arrived at the conclusion that the felspar is oligoclase, and 

 not albite (as is more usual in such intergrowths). 



The presence in such large quantities of this peculiar form of 

 perthite should serve as a valuable means of identifying doubtful 

 cases of erratics from this source. 



The micas include both muscovite and biotite, the former pre- 

 dominating, except in the local variety found at Wabberthwaite. 

 The muscovite is usually quite fresh, but the biotite is often repre- 

 sented by chlorite, or near the great ironstone-veins by skeletons of 

 haematite together with a little calcite. 



The granite is remarkably free from accessory minerals, even 

 apatite being exceedingly rare. In one specimen only I found a small 

 quantity of tourmaline, but in several there was a little magnetite 

 which, being associated with decomposed biotite, I conclude to be 

 secondary. 



Order of Separation of the Minerals. 



In the normal granite the micas appear to have been the earliest 

 secretion of the magma and to have separated simultaneously, being 

 usually found in parallel intergrowth. The next secretion was 

 apparently in most instances the oligoclase, which was followed by 

 the perthite, orthoclase, and quartz in that order, although, as will 

 be shown later, simultaneous separations and even reversals of some 

 of the terms in the sequence occurred at least locally. 



As a general rule, the granite is chiefly made up of quartz and 

 perthite, the micas being present only in small quantity. 



Micropegmatitic structure occurs, though rarely, and it is of a 

 coarse-grained type very unlike that of the Ennerdale Granophyre. 

 It usually consists of quartz and orthoclase, but instances are not 

 wanting in which it contains perthite in place of orthoclase (6). 

 In some instances a band of micropegmatite surrounds a crystal of 

 orthoclase, having grown on to it in zonal fashion, so that the quartz- 

 granules in the two portions of a Carlsbad twin are differently 

 oriented as regards their extinction (9). This zonal arrangement 

 of micropegmatite shows that, in this instance, its formation 

 immediately succeeded that of the orthoclase. 



Micropegmatite is usually the last secretion of a rock in which 

 it occurs ; but, in this case, the orthoclase-crystal, with its zone of 

 micropegmatite, is surrounded by a ground-mass of quartz and felspar 

 in a fine mosaic, although exhibiting no trace of micropegmatitic 



