part 2] THE SHAP MINOR INTBUSIONS. 135 



always readily distinguished from the xenocrysts. A section from 

 a joint-plane weathering green contains amphibole almost to the 

 entire exclusion of biotite, although this mineral is absent from the 

 remainder of the rock. 



Another type contains amphibole in the ground-mass, in addi- 

 tion to biotite. These rocks are dark, with a greenish tinge 

 due to the amphibole. They contain pink orthoclase- and quartz* 

 xenocrysts, together with biotite, hornblende, and plagioclase. 

 Microscopically the ground-mass has a patchy appearance, sugges- 

 tive of an imperfect mingling of material. The lighter patches 

 are of felspar and quartz ; orthoclase may be most abundant, 

 sheaf-like aggregates being then produced, or the felspar may be 

 wholly plagioclase with interstitial quartz. The dark minerals are 

 similarly bunched together. Hornblende may predominate (as in 

 one example from the upper part of Stakeley Valley), or biotite 

 may be more prevalent : but sections from the same dyke do not 

 show any decided uniformity. The hornblende maybe the pale- 

 green fibrous variety resembling that which now forms the corrosion- 

 ring of the quartz, often in tufted aggregates with brilliant 

 polarization ; or the colourless variety may be present in the pilitic 

 pseudomorphs. These rocks are allied to the vogesites rather than 

 to the minettes, and are pilitic vogesites or pilitic biotite- 

 vogesites modified by acid xenocrysts. 



A variety closely resembling the minettes of the district occurs 

 in various parts of Stakeley Valley and near Gill Farm. Essenti- 

 ally the rocks consist of biotite and felspar, the latter principally 

 orthoclase-prisms grouped in fanlike aggregates. Pseudomorphs 

 of calcite and chlorite are present, together with vesicles, but 

 hornblende is entirely absent. Either augite-minettes or possibly 

 even pilitic minettes may be represented in this group. 



These examples illustrate the difficulty attending any attempt 

 to classify hybrids. Judged by the density 270 to 2 - 73, the part 

 played by ferromagnesian minerals, the iron content, and the con- 

 stituents of the ground-mass, they should be classed as lampro- 

 phyres. But the xenocrysts, never entirely absent and often present 

 in great number, are granitic, and it is not always easy to strike 

 a balance between the two. To term them lamprophyres is to 

 ignore the granitic element; to describe them as porphyrites is 

 to ignore on the one hand the lamprophyric groundwork, and on 

 the other, the xenocrysts. The use of the general term of ' quartz- 

 porphyrite ' is scarcely warranted, as biotite is the more frequent 

 porphyritic element and is moreover native to the magma ; while 

 a consistent application of this mode of nomenclature would lead 

 to such cumbersome forms as 'orthoclase-biotite-hornblende-quartz- 

 porphyrite,' with variations, which obscure the real character of 

 the rocks. 



The intrusions of the mixed series are thus the result of a com- 

 plicated differentiation, and are marked by increasing basicity in 

 an increasingly important ground-mass, retarded by the addition of 



