part 2] THE SHAP MINOR INTRUSIONS. 125 



A comparatively narrow dyke in Wasdale Beck, almost hidden 

 from view, presents some unusual features. It is coarsely crystal- 

 line in appearance, and its structure granitoid rather than por- 

 phyritic. Light grey in colour, the rock is an aggregate of quartz 

 and felspar, with unevenly-distributed Hakes of brown biotite. 

 Towards the margin of the intrusion green specks appear, and then 

 become abundant ; biotite steadily decreases, and is ultimately 

 replaced by the green mineral. A specimen from a big block in 

 the stream, since disappeared, has a specific gravity of 2*617 (the 

 highest of all the acid rocks), while a specimen from the margin 

 gives 2 658. 



Normally the rock is a medium-grained agoresrate of light- 

 coloured minerals, reproducing some of the features of the granite 

 and consisting mainly of quartz, oligoclase, and perthite. Occurring 

 sparingly, and intergrown in one instance with biotite, is diopside : 

 this towards the margin becomes increasingly abundant, gradually 

 taking the place of biotite, and occurring both in irregular crystal- 

 line form and as granular aggregates. Its presence points to the 

 absorption of calcareous material by the igneous magma, material 

 which had already undergone metamorphism by the granite. Asso- 

 ciated with it are irregular grains of reddish-brown sphene and 

 abundant apatite. Further evidence of assimilation is seen in the 

 zoned oligoclase, of which the outer zone is more basic than the 

 core. The plagioclase and diopside are extremely irregular in form, 

 having consolidated at a late stage. An excess of quartz in places 

 has doubtless been obtained from the invaded flags. The dyke 

 was probably intruded before the country-rock had cooled, and 

 thus was capable of incorporating and assimilating the material 

 into which it was injected. This would account for the relatively 

 coarse texture in a dyke of such small dimensions, because, as a 

 rule, even massive dykes of this group have failed to produce any 

 appreciable effect on the ground-rock. An early period must, 

 therefore, be assumed for its intrusion. 



The junction-rock is interesting, as showing a higher grade of 

 alteration than the surrounding rocks, metamorphism by the dyke 

 subsequent to that by the granite involving a recrystallization to a 

 mosaic of clear felspar and quartz through which are scattered 

 bright-green granules of diopside. 



V. The Intermediate Intrusives. 



Rocks differing considerably in character are grouped for con- 

 venience under this head. Many of them are porphyrites ; but a 

 more general term is used, in order that certain more basic mem- 

 bers of one group may be included for discussion along with the 

 porphyrites with which they are closely connected. These intru- 

 sions are found in many parts of the area, and exhibit striking 

 points of difference. Some are of more or less restricted occur- 

 rence, others cover a wider field. Generally, however, they fall 

 into one of two well-defined groups : (a) the Potter-Fell type, 



