298 MESSRS. A. HARKRK AND J. E. MARK ON 



water would be almost the only chang:es involved in the conversion 

 of the foroicr into the latter mineral, and this appears to have been 

 the usual mode of alteration in the rocks in question. To produce 

 hornblende, however, would require the taking up of lime, as well 

 as silica, and the distribution of this mineral in patches and streaks 

 in the metamorphosed rocks, and particularly withiu the vesicles, 

 seems to show that its formation, instead of biotite, depended upon 

 the presence of calcite in the immediate neighbourhood of the 

 delessite. Augite contains much more lime than hornblende, and 

 its mode of occurrence as a metamorphic mineral accords well with 

 our suggestion. The veins of pure augite [759] may be taken as 

 representing veins of crystalline calcite traversing the weathered 

 andesite before its metamorphism. The mineral is identical with 

 that to be described below as one of the most abundant silicates in 

 metamorphosed rocks of the calcareous group, and is presumably a 

 variety rich in lime. 



With respect to the sphene, it is not easy to say in what form 

 the titanic acid existed before the metamorphism. Ilmenite does 

 not appear to be a common constituent of the original andesites, 

 though it occurs rather abundantly, with secondary translucent 

 sphene, in some of the ashy beds [766]. The sphene, a lime- 

 bearing mineral, naturally occurs in association with the hornblende 

 and colourless augite rather than with the mica, but the titanic acid 

 may have been distributed uniformly through the weathered 

 andesite and be now partly contained in the last-named mineral. 

 The biotite of Miask is known to have 4*73 per cent, of titanic 

 acid *, and this substance is beginning to be recognized as a wide- 

 spread constituent of the brown rock-forming micas. 



The magnetite, again, is mostly found in association with horn- 

 blende, but it is possible that the other parts of the rock contain as 

 much iron, which is there incorporated as part of the brown mica. 

 The flakes of the latter mineral are too minute to allow of any 

 precise study which might determine whether they should be 

 referred to biotite, haughtonite, or lepidomelane. The strongly 

 pleochroic brown mica of thermo-metamorphic rocks (Hornfels^ &c.) 

 is usually stated to be biotite, but we shall allude to this point 

 again below. 



The specific gravity of a highly-metamorphosed vesicular andesite 

 from near the northern border of the granite was found to be 

 2'SOO, the figures for the non-metamorphosed rock in Stockdale 

 being 2-736. We shall see that the metamorphism of the rocks 

 around the Shap granite is in general accompanied, as in this case, 

 by a condensation of bulk. 



In this place we may most conveniently notice the metamorphism 

 of certain ashes, fine agglomerates, &c., which are closely associated 

 with the andesitic lavas. The fragmental volcanic rocks, having 

 usually a heterogeneous constitution, do not admit of any very strict 



■* R. Schlapfer, ' Rech. sur la compos, des micas etdes chlorites,' Schaffhausen 

 (1 889). See also Koch, Zeitschr. deutsch. geol. Gesellsch. vol. xli. (1889) p. 16.5 ; 

 W. M. Hutchings, Geol. Mag. (1890) pp. 272, 273. 



