134 MR. J. MORRISON ON [vol. lxxiv, 



giving colour to the rock, or it mav be entirely absent. It is 

 probably secondary after augite, though the latter is never present 

 in an unaltered condition. 



More interesting, however, is the occurrence of pseudomorphs 

 of which the original mineral has disappeared. Occasionally a 

 shape suggests augite, at other times olivine, though neither 

 mineral is present in a fresh condition. Some of the pseudomorphs, 

 consisting of a mass of fibrous amphibole of the kind already 

 described, may represent augite. Others show a diversity of 

 secondary minerals, and (though often rounded) have at times the 

 outlines and sharp terminations of olivine. Commonty they consist 

 of an aggregate of colourless amphibole-prisms grouped without 

 any definite arrangement, which may either occupy the whole of 

 the pseudomorphs or may be associated with other secondary 

 minerals. They resemble the ' pilite ' pseudomorphs after olivine 

 described by F. Becke. 1 The amphibole has the characters of the 

 grammatite of B. Doss. 2 It is colourless and highly birefringent, 

 cross-sections showing the usual amphibolic cleavage. Generally 

 the prisms form a tangled mass ; occasionally they lie along the 

 vertical axis of the olivine, or there may be some feeble trace of 

 zoning. Chlorite sometimes fills up the spaces, but pseudomorphs 

 of amphibole alone are frequently crowded with secondary magne- 

 tite. Secondary biotite is sometimes present, and is distinguished 

 from the original biotite in the body of the rock (often clinging 

 to the exterior of the pseudomorph) by the absence of the rutile 

 mesh which usually accompanies the decomposition of the latter, 

 and b}^ the greenish-brown colour. A colourless interior is occasion- 

 ally surrounded by a mixture of amphibole and new biotite similar 

 to that described by Doss ; but his suggestion that dynameta- 

 morphism has played a part in their production has no application 

 to the present examples. Aggregates of highly-polarizing scales 

 resembling talc are sometimes associated with chlorite and amphi- 

 bole. Other pseudomorphs consist of calcite and chlorite with a 

 little iron, while in one intrusion they are composed almost entirely 

 of biotite-flakes. 



A dyke of the pilitic type occurs on the north side of Stakeley 

 Valley, breaking through and incorporating the Coniston Flags. 

 Centrally the rock is dark grey, marginally it is black and compact. 

 White orthoclase-xenocrysts are conspicuous, but seldom exceed a 

 centimetre in length ; quartz is less abundant, and oligoclase is 

 rounded. Biotite now assumes great importance, both as pheno- 

 crysts and in the matrix. In section, the bulk is seen to consist of 

 a fine-grained ground-mass of biotite and felspar. Pseudomorphs 

 are abundant : some showing olivine shapes consist of amphibole 

 and chlorite with grains of iron-ore, thus resembling the original 

 'pilite.' Quartz-grains incorporated from the invaded rock are 



1 ' Eruptivgesteine aus der Gneissforination des Niederosterreichischen 

 Waldviertels ' Tscherm. Min. Petr. Mitth. vol. v (1883) p. 163. 



2 ' Die Lamprophyre & Melaphyre des Plauen'schen Grundes bei Dresden ' 

 ibid. vol. xi (1890) p. 17. 



