THE SHAP GRANITE AND ASSOCIATED ROCKS. 311 



diiction of silica in connexion with the metamorphism. None of 

 the altered calcareous strata can be followed for more than about 

 fifty yards along their strike, and the exposures, which are all 

 within 300 yards of the granite -boundary, show complete meta- 

 morphism in every case. The next appearance of the Calcareous 

 Breccia towards Great Yarlside, about three-quarters of a mile from 

 the granite, shows little or no alteration. As the limited nature of 

 the exposures thus precludes any attempt to trace the gradual 

 changes in particular beds, we shall content ourselves with describ- 

 ing the constitution of the chief types of metamorphosed limestones 

 found in the sections mentioned. 



Beginning with the representative of the Lower Limestone, the 

 most interesting type to be noticed is that consisting mainly of 

 idocrase with lime-garnets and other lime-bearing silicates. In 

 hand-specimens the idocrase is often seen to compose the bulk of 

 the rock, and is readily identified by its light-brown colour, rather 

 imperfect cleavage, and resinous lustre. It forms a framework in 

 which the garnets, &c. are embedded, and usually presents no 

 crystal-outline, nor does it then show the concentric shell-structure 

 found in many idocrases. The specific gravity of a typical specimen 

 of the rock was found to be 3*476, which answers to idocrase with 

 a smaller quantity of lime-garnet. In some parts of the rock the 

 idocrase shows crystal-boundaries, viz. the prism (m), pyramid {y), 

 and basal (c) *. 



In thin sections the idocrase appears as large shapeless plates en- 

 closing the garnets and other constituents in ophitic fashion, and, 

 as is often the case in minerals having this mode of occurrence, the 

 cleavage-cracks are not well developed (see PL XII. fig. 1). The 

 mineral gives bright polarization-tints, which vary slightly in dif- 

 ferent parts of a crystal. Idocrase is usually stated to give very 

 low tints: Rosenbusch says that the birefringence rarely exceeds 

 •0015 ; Michel-Levy and Lacroix give '0015 as the mean and, ap-. 

 parently, '002 as the maximum. Prof. Brogger, however, found 

 brightly polarizing idocrase in the metamorphosed calcareous rocks 

 of the Christiania district, and a slice [1042] of the Monzoni ido- 

 crase shows the same character. The idocrase of our rock contains 

 numerous inclusions, mostly minute granules of pyroxene (?), and 

 sometimes crowds of little needle-shaped crystals which we have not 

 identified [1169]. 



The garnets occur in dodecahedra, up to ^ or sometimes ^ inch 

 in diameter, which are often isolated in the exposed outcrop of the 

 rock, owing to weathering. The crystals have lustrous faces, with 

 the somewhat greenish-yellow colour common in the lime-alumina- 

 garnets (grossularite). In thin slices the mineral shows some in- 

 teresting features. It is seen to be birefringent, and to present in 



* Ml'. W. M. Hutchings informs us that lie has measured one of these 

 crystals on the goniometer. He finds the angle between the basal plane (c) 

 and the unit pyramid (_?/) to be between 37° and 38 '^. Miller gives, for ido- 

 crase, 37° 7^ Mr. "Hutchings also states that the optical properties are those 

 of normal idocrase. 



