344 LIEUT.-GEISr. C. A. M c MAnON ON ROCKS OP [Aug. 1 894, 



qnartz-veins. They have somewhat the appearance of a metamorphic 

 sedimentary rock. 



Under the microscope this rock, at first sight, has very much the 

 character of a metamorphosed sedimentary rock, and seems to consist 

 of a granular mixture of quartz and flaky greenish-brown mica. On 

 closer examination, however, particularly with somewhat high 

 powers, it is seen that the matrix is by no means all composed 

 of quartz and mica, and that cryptocrystalline felsitic matter 

 predominates over the clear grains. All the water-clear grains, 

 moreover, are not quartz, for in some I obtained in converging 

 polarized light traces of biaxial interference-figures. In this granular 

 groundmass are scattered crystals of quartz and felspar, some of 

 which are idiomorphic and yield sharp crystallographic outlines. 

 These idiomorphic crystals afford, I think, decisive evidence of the 

 character of the rock. I have seen a groundmass of identically the 

 same structure in some undoubtedly igneous rocks, as, for instance, 

 in some of the laccolites of the Henry Mountains, U.S.A. 



Slice No. 14 contains much, and No. 17 a little, blue schorl 

 in ophitic aggregations, its crystalline form being interrupted by 

 the granules of the groundmass. Some of the large quartzes contain 

 numerous liquid cavities with bubbles. 



No. 15. The main constituent of this rock is a purple-brown glass, 

 containing countless grains of magnetite arranged in fluxion-lines, 

 microlites, and small prisms of felspar. The microlites have straight 

 extinction, but the small prisms are in part plagioclase and in part 

 orthoclase with binary twins. The glassy groundmass is vesicular, 

 the oval vesicles being now stopped with red iron oxide, magnetite, 

 and a colourless isotropic substance. The slice contains a portion 

 of an included fragment of a fine-grained sedimentary rock. 



There are numerous lacunas in the brown glass, and they contain 

 water-clear felspar, a network of green hornblende-prisms, and some 

 massive hornblende without crystallographic shape. Congeries of 

 green hornblende-prisms are also scattered through the glass. 



No. 16. The groundmass of this specimen is substantially the 

 same as that in Nos. 14 and 17, but it contains a much larger 

 amount of pale-greenish mica, in scales and fibres. 



The porphyritic crystals of quartz and felspar are extremely 

 abundant in this specimen. They all show very distinct remains of 

 crystallographic outlines ; but they are, especially the quartzes, 

 deeply corroded by the groundmass. The large felspars, on the 

 other hand, have suffered very much from the formation of quartz 

 and water-clear felspar (in some cases it is certainly quartz) in their 

 interior. Very often this alteration has gone so far that the crystals 

 have acquired a very decided granophyric structure. That this .is, 

 in these rocks, the result of secondary alteration I have no doubt. 

 In the felspars that exhibit this structure small circular rings of 

 quartz, or water-clear felspar, marking the passage of liquids, or gases, 

 through the rock, have been left as evidence of their former presence. 

 The centres of these annular bodies are filled with secondary 

 mica, and they are sometimes fringed with the same mineral. 



