W. M. Hutchings — Rocks of Great Whin Sill. 73 



averaging about to-q-o i'lch ia diameter, but in souie cases reaching 

 xo^o iticli and a little over. 



At some parts of the slides these garnets are packed so close that 

 scarcely anything else is visible. They vary from colourless to 

 yellow and greenish, and some are a rich red-brown. Often the 

 centre is coloured, and the outer rim is colourless. Augite occurs 

 plentifully with the garnet, in good-sized crystals and large irregular 

 complex grains. It is all perfectly fresh, and some of it is of a very 

 decided green colour, and slightly dichroic. Both garnets and augite 

 come right up to the contact-line, and in one slide may be seen lines 

 of very small augite crystals, clearly of contact-origin, growing out 

 from the edge of the Whin, like the teeth of a saw, into the lime- 

 stone. A very pale hornblende in slender needles is also present at 

 some parts ; epidote and sphene are well represented, and there is 

 a good deal of recrystallized quartz, which frequently encloses garnet 

 and augite. The remaining calcite of the limestone is completely 

 recrystallized. There are often large fields of one uniform grain of 

 it, with numerous garnets and augites contained in it. 



In the mosaic of recrystallized quartz in these highly calcareous 

 rocks one may often suspect felspar to be present, Ijut without being- 

 able to make sure of it, owing to absence of cleavages and the 

 impossibility of making reliable optical tests. In one specimen from 

 near Dunstanburgh, however, an altered limestone shows numerous 

 small crystals, together with more or less irregular grains, of well- 

 cleaved fully-individualized felspar. Some few of the crystals even 

 allow of identification, with much safety, as anorthite or a closely- 

 allied species (sections with parallel cleavage, extinctions 30°-4:0°, 

 "with emergence of good axial bar inside the field). They lie in 

 amongst very coarse-grained recrystallized calcite, near the junction 

 with the Whin. It will be seen later on that some of the altered 

 shales contain abundant new felspar. The above specimen shows 

 also a few garnets, some epidote, a good amount of recrystallized 

 quartz, together with a considerable amount of wollastonite, mainly 

 in tufts and bunches of often sheaf-like, radiating fibres, with here 

 and there bits of sufficient size for the application of optic tests. 

 This mineral appears to be not of frequent occurrence in these 

 rocks. Indeed, it may be noted that, so far as concerns the lime- 

 stones which are reasonably free from any admixture except silica, 

 there seems seldom to be any reaction between the lime and the 

 silica. Calcite and quartz recrystallize side by side, and it is rare 

 to see in these particular rocks any formation of wollastonite, or of 

 any calcareous hornfels-like products, such as are more frequently 

 encountered round granite-contacts. 



Sometimes there were little bands of sandstone in the pure lime- 

 stone, quite close to the contact. A specimen from near the Koman 

 station of Borgovicus, close to the Whin, is sliced so as to show both 

 recrystallized limestone, very saccharoidal, and sandstone converted 

 into a well-cemented quartzite, with some new felspar among the 

 interstitial matter. The division-line of the two products is very 

 clear and sharp, and tliere has not been a trace of action between them. 



