THE SHAP GRANITE AND ASSOCIATED KOCKS. 313 



appear from the accounts of different writers to be very character- 

 istic of such rocks. Another British example is aifordcd by the 

 Carboniferous calcareous shales in contact with the large dyke at 

 Plas Newydd on the Menai Straits. The garnets at that locality 

 have a zonary structure, and show the same tyjje of polysynthetic 

 grouping as those of the Wasdale Head rock [149]. 



In the most common type of the metamorphosed Lower Limestone, 

 doubly-refracting garnet and idocrase build most of the mass. 

 Among other minerals met with are pyroxenes similar to those to 

 be described in the Upper Limestone. A colourless augite is the 

 most common, and is evidently the same as that described below. 

 It is sometimes abundant in crystalline patches showing augite- 

 cleavage and even twinning [1207], and the minute granules, giving 

 bright polarization-tints, which often crowd both garnet and idocrase 

 are perhaps the same mineral. These little granules have the 

 rounded or " globulitic " appearance which has been commented on 

 by Prof. Brogger in similar rocks in Norway. 



Another mineral not infrequently found is tremolite, which forms 

 little veins and patches, and encloses imperfect crystals of light- 

 brown sphene [1170]. Anorthite is sometimes to be identified. 

 Quartz apparently does not occur, except in narrow veins evidently 

 representing little cracks. 



Another type met with in the altered Lower Limestone shows in 

 hand-specimens a dull-white ground studded with round light-brown 

 spots, up to about -^ inch in diameter, and more irregular pinkish- 

 brown patches of similar size. The round spots are garnets, which 

 in slices are found to be isotropic, and the less regular patches are 

 evidently imperfectly-separated crystals of the same mineral. Both 

 contain a large amount of enclosed material similar to their matrix, 

 which seems to be usually a very finely granular aggregate of wol- 

 lastonite, augite, &c. [951]. It may be noted that these isotropic 

 garnets are of a browner colour than the doubly-refracting ones . 

 (essonite) ; but there is no reason to suppose that the isotropic 

 character is connected with chemical composition. The isotropic 

 garnets are never in such perfect crystals as the others, and often 

 seem to have been arrested in an incomplete state of development. 



The metamorphosed representative of the Upper Limestone is a 

 compact porcellanous-looking rock of pale bluish-grey or greenish- 

 grey colour, closely comparable with lime-silicate hornstones from 

 the Harz and other regions. On closer examination, it often shows 

 a rather mottled appearance on a small scale, some parts being 

 greenish and giving evident indication of crystalline structure. 

 The lower beds, representing the Calcareous Breccia, enclose nume- 

 rous angular, subangular, and rounded fragments similar to those 

 seen in the unaltered strata in Blea Beck. Some of these fragments 

 are of dark colour and dull appearance, but the majority have a grey 

 horny aspect, and are at once recognized as the usual rhyolite-frag- 

 ments of these beds, though they appear to have suffered some meta- 

 morphism, and their boundary against the enclosing matrix is not 

 always perfectly sharp. 



Q.J.G.S. No. 187. - z 



