part 2] THE SHAP MINOB INTRUSIONS. 127 



Nearer the granite, on the fells of Fawcett Forest, a series of 

 somewhat similar intrusions occur. The rocks are finely crystal- 

 line to compact, even-grained rather than porphyritic, the felspars 

 smaller and more uniform in size, and oligoclase preponderates in 

 the ground-mass, which is here definitely micro-graphic. Rounded 

 quartz-grains with a spherulitic border appear in another dyke of 

 this group, 3 miles south-south-east of the granite. They are 

 associated with phenocrysts of oligoclase-andesine, in a matrix 

 of which the dominant element is a spherulitic intergrowth of 

 orthoclase and quartz. This intrusion is therefore more acid than 

 those described, and its occurrence in the north-north-westerly 

 belt with approximately the same trend is of interest. 



As the extreme acid variety of the type now described a dyke 

 may be mentioned, situated within the same belt, but occurring in 

 the outer area, near Lummer Head, about 7 miles from the granite. 

 It is a pale-buff-coloured medium-grained felspathic rock, with a 

 distinct pearly lustre due to the felspar-phenocrysts and to colour- 

 less mica. In thin section quartz appears as a phenocryst, in 

 addition to oligoclase and orthoclase. The rock has affinities with 

 the porphyries rather than with the porphyrites. 



(b) The Mixed Series. 



These intrusions are restricted to a somewhat limited area in the 

 immediate neighbourhood of the granite. Their width is often 

 more or less conjectural — some indeed being little more than a 

 series of boulders ; but generally they are of smaller dimensions 

 than the acid intrusions, and, though never attaining any degree of 

 prominence in the field, they present some unique features and are 

 of considerable petrological interest. 



The rocks are usually dark, micaceous in character, and por- 

 phyritic in structure, and represent a fairly wide range of com- 

 position. The characteristic feature is the presence of the large 

 orthoclase-felspars of the granitic type which form a striking 

 contrast to the dark groundwork of the rock, inviting almost 

 inevitably comparison with the basic patches of the granite. 



The study of these intrusions in the field and under the micro- 

 scope appears to give the clue to their origin. The large porphy- 

 ritic orthoclases are prominent in all the rocks in varying degree ; 

 but within this limited area their relative abundance does not 

 depend upon the distance of any given intrusion from the granite. 

 Outside the area their occurrence, in other than the acid dykes, is 

 exceptional, the mixed rocks giving place to lamprophyres con- 

 taining small rounded quartz-grains in unusual association. Traced 

 outwards from the granite, therefore, the felspars abruptly vanish, 

 the quartz that still remains being reduced to small, often micro- 

 scopic, dimensions. The intimate association of these intrusions 

 with the granite thus indicated is emphasized by the fact that 

 the rocks of the series are entirely confined to the narrow belt 

 previously mentioned. 



