Vol. 60. QTTARTZIXE-DYKES IN MOUXTAIX-LMESTOXE. 365 



a brown colour. In a hand-specimen the partly-doloraitized rock 

 is speckled with brown spots, which with a lens are seen to consist 

 of crystals and groups of crystals of dolomite. 



The floor and faces of the quarry are traversed by vertical veins 

 or dykes of calcite, fluorspar, barytes, calcareous sandstone, and 

 quartzite. On my first visit, I noticed several blocks of siliceous 

 rock, which I was informed had been obtained from a so-called 

 bastard limestone on the south-eastern side of the quarry, and 

 that 20 tons of this rock had been removed. The place from 

 which it was obtained is now covered with soil and vegetation, 

 and consequently no outcrop is visible. The bastard limestone or 

 quartzite probably filled a fissure, pipe, or swallow-hole in the 

 limestone. 



PI. XXX, fig. 1, shows one of these dykes traversing the face 

 of the quarry. It varies from 2 to -1 inches in width, and consists 

 mainly of quartzite, but barytes and calcite are also found in it in 

 places. It is darker than the limestone and very hard. 



PL XXX, fig. 2, shows another dyke, which attains a width of 

 19 inches, and consists of a hard sandstone or quartzite. 



II. Petrography. 



(1) The Quartzite-Dykes. 



A number of thin slices of the quartzite and of the limestone 

 were examined under the microscope, but it will be sufficient for 

 the purposes of this paper to refer to the following. 



Xo. 1316, 1 from the 4-inch vein, examined under the microscope, 

 cannot be distinguished from a quartzite. It consists of angular and 

 detrital quartz-grains with enclosures, a few small pebbles or grains 

 of felspar, and a few shreds of mica. The grains are cemented by 

 silica, and sometimes by calcite ; the interspaces are often filled with 

 a secondary growth of quartz around the grains. (See PI. XXXI, 

 fig. 1.) The limestone (1315) in contact with the quartzite is fine- 

 grained and crystalline. 



A specimen from a softer dyke (1318) consists of quartz, a small 

 quantity of mica, and traces of monoclinic felspar. The quartz - 

 grains are both angular and well-rounded. The formation of 

 secondary silica in optical continuity with the original graius is well 

 shown, and calcite is also present. (See PI. XXXI, fig. 2.) 



A specimen (1235) from another dyke consists of quartz-grains 

 and a few felspars cemented by calcite (see PI. XXXI, fig. 3). 



In the south-eastern face of the quarry, near where the bastard 

 limestone was worked, is another dyke of quartzite, which extends 

 to the topmost exposed bed. The rock (1086) consists of quartz, 

 with a small quantity of felspar and mica. Some of the felspars 



1 These numerals throughout refer to the numbers of the slides in the 

 writer's collection. 



