part 2] THE SHAP MINOR INTRUSIONS. 133 



for the first time, clearly secondary, and probably pseudoinorphous 

 after augite. The presence of pyroxene in the basic patch and its 

 absence from the enclosing rock raises the question of the origin 

 of the patch. Incorporation of the country-rock is out of the 

 question. The abundance of minerals common to the host, with 

 the addition of pyroxene which is characteristic of a lower portion 

 of the magma, suggests the source. The higher density, greater 

 basicity, and the fact that both were liquid together (shown by 

 the absence of a sharp boundary and the commingling of material) 

 point to the same conclusion. It is a portion of the underlying 

 magma which, becoming involved in the intrusion, gravitated to 

 the lower edge of the dyke, and finally crystallized as a more 

 or less independent mass. 



A further stage in the corrosion of the xenocrysts and the 

 gradual increase in basicity of the matrix is seen in a dyke not far 

 off. The xenocrysts are less abundant, the compact matrix is 

 darker and of relatively greater importance, and the rock is gene- 

 rally less porphyritic. Microscopically the principal differences 

 are the greater abundance of mica of both generations, the appear- 

 ance of amphibole in the body of the rock, and the presence of 

 reaction-rims to the quartz. The rims consist of a confused tangle 

 of flakes, with no definite orientation, and increase in width at the 

 expense of the quartz. Some of the amphibole-aggregates there- 

 fore probably indicate the position of former quartz-grains. 



In these three dykes, all occurring north of the granite, with 

 specific gravities respectively of 2*6-12, 2654, 2*684, successive 

 stages of progressive basicity are exemplified, and the hybrid 

 character becomes increasingly evident. Attempts to classify them 

 on the basis of mineral constituents alone would lead to confusion, 

 as it would be difficult to assign to them a place in any scheme. 

 The difference between the corroded orthoclase and quartz on the 

 one hand, and the idiomorphic plagioclase and biotite on the other, 

 is fundamental, and has to be taken into account. Apart 

 from the xenocrysts the rocks described are biotite-porphyrites, 

 consequently they must be regarded as biotite-porphyrites modified 

 by the inclusion of acid xenocrysts from a more acid but allied 

 magma. In mineral composition and character the third of the 

 series represents an approach to the lamprophyric type. 



Intrusions showing a still further advance in basicity, and 

 therefore presumably from a lower part of the magma, occur south 

 of the granite. They are more compact, usually dark grey or 

 reddish brown, and the structure at times is almost panidiomorphie. 

 Considered apart from the xenocrysts, which are smaller than 

 before, though equally if not more conspicuous, many of the rocks 

 are lamprophyres, and frequently biotite is the commonest and 

 only idiomorphic mineral of the larger generation. Hence they 

 are lamprophyres modified by acid xenocrysts. 



Microscopically they show certain peculiarities. Amphibole 

 increases in amount, and vies with biotite in importance, but with 

 no claim to be considered of primary origin. It may predominate, 



l2 



