part 2] THE SHAP MINOK INTRUSIONS. 131 



of intrusions ; but at times it is much bent and fractured, has 

 a deep red-brown colour as in the lamprophyres, and breaks down 

 commonly into a mass of chlorite crowded with rutile-needles — a 

 type of alteration very uncommon in the preceding groups, but 

 characteristic of this series. Lenticular calcite and chlorite are 

 fairly frequent alteration-products in some varieties. Lamellar 

 twinning is not unusual. Always abundant, biotite gradually 

 increases in importance, and ultimately becomes the dominant 

 phenocryst. 



Absent from the upper rocks, but subsequently becoming asso- 

 ciated with the biotite. is a pale amphibole, either colourless or 

 faintly tinged with green, the pleochroism being : X almost colour- 

 less, Y yellowish-green, Z faint green. Typical hornblende shapes 

 are never seen, but ragged cross-sections show the usual amphibolic 

 •cleavages. The extinction seldom exceeds 18°, polarization is 

 brilliant, and lamellar twinning is quite common. The crystals 

 often consist of aggregates of actinolitic needles, and the appear- 

 ance is always suggestive of a secondary origin. With the incoming 

 •of amphibole the quartz-xenocrvsts begin to show a reaction-rim 

 of the same substance, which increases in width as the quartz 

 diminishes in size. Very rarely an augite shape is suggested, 

 though never sharply defined, and it is probable that some of the 

 •amphibole may replace that mineral. There are strong grounds, 

 however, for believing that some of it has been derived from biotite. 



The abundance of sphene is in marked contrast to its rarity in 

 the acid intrusives, and to its practical absence from the Potter- Fell 

 group. It is constantly present, not only in the form of wedges, 

 but also as irregular grains. Twinning on 110 is common, but 

 lamellar twinning parallel to 221 has also been observed. Alteration 

 leads to a mass of calcite and rutile bordered by packed grains of 

 •opaque iron-oxide, and occasionally accompanied byanatase. 



Epidote is always present ; a more interesting occurrence, how- 

 ever, is allanite, which is found in several of the dykes, though 

 never in abundance. It occurs mainly in small irregular grains, but 

 occasionally forms Ion": narrow crystals about 25 mm. in length, 

 elongated along the b axis to about eight times the width. Some- 

 times it is surrounded by epidote in parallel orientation. The 

 •colour is somewhat patchy, but the pleochroism gives in order 

 yellowish-green, brownish, brownish- vellow. Transverse zoning is 

 well marked, -while apatite and zircon are frequently enelosed. 

 It is interesting to note that allanite is 'also found in the granite, 

 •a section in the Sedgwick Museum Collection containing an 

 elongated crystal similar to those just described. 



The remaining accessory minerals include abundant apatite, 

 zircon, pyrrhotite, ilmenite, and some secondary pyrites. Pyrrhotite 

 is fairly common in certain rocks, and, with biotite, forms occa- 

 sionally small basic aggregates. Carbonates are freely disseminated 

 through the more basic rocks of the series, and accompany quartz 

 in the vesicles. The carbonates which form so prominent a feature 

 •of the lamprophyres generally have been referred variously in 



Q.J.G.S. Xo.*294. l 



