384 



increase in size as the junction is approached. They are, as a rule, 

 devoid of external crystalline form. The dark mica is highly 

 characteristic of contact-rocks. It often occurs in minute irregular 

 overlapping scales. Its colour is usually less marked than that of 

 the black mica in igneous rocks and it rarely if ever shows definite 

 boundaries in the prismatic zone. When inclusions of zircon occur 

 they are generally surrounded by pleochroic borders as in the igneous 

 rocks. The brown mica takes the place of chlorite and to some 

 extent also that of the sericite of the original rock. It may, by subsequent 

 alteration, again give rise to chlorite, as in some of the Cornish 

 contact-rocks. White mica and brown mica frequently occur intergrown 

 with each other. The secondary mica in some of the Cornish rocks 

 is often of a reddish brown colour. 



Wollastonite. This mineral is especially characteristic of calcareous 

 rocks. It is monoclinic but elongated in the direction of the ortho- 

 diagonal axis, so that longitudinal sections give straight extinction. In 

 this respect it resembles epidote. Cross sections of good crystals are 

 six- or eight-sided and are bounded by the traces of the forms (001), 

 (100), (102) and (101). The most perfect cleavages are parallel to 

 001 and 100. The angle between these two faces is 95 30'. The 

 optic axial plane is the clino-pinacoid and it is therefore at right 

 angles to the direction of elongation as in epidote. The extinction 

 referred to the trace of 001 in a clino-pinacoidal (cross) section is 

 32. Refraction and double-refraction are both high, but not so high 

 as in the normal monoclinic pyroxenes. In thin sections the mineral 

 is colourless. 



Pyroxene. Normal pyroxene is often developed in rocks containing 

 more or less calcareous matter. It is frequently found in limestones 

 and in lime-silicate-hornfels in association with wollastonite. It is 

 usually colourless in thin section and occurs in the form of more or 

 less rounded grains in which, however, the characteristic cleavages and 

 optical properties may be recognized. 



Hornblende-group. Actinolite and tremolite occur in certain rocks 

 near granite masses. The occurrence of calcareous matter in the 

 original rock is necessary for their formation, so that they are not 

 developed in purely argillaceous sediments. 



Felspar. The occurrence of felspar as a contact mineral was first 

 demonstrated by Mr. ALLPORT (I) in the case of a rock occurring at 

 Enniscorthy, Co. Wexford. Professor BROGGER has since recorded the 

 occurrence of felspar (mostly plagioclase) in several contact rocks 

 from Southern Norway. In the metamorphosed limestone-nodules 

 (stinkkalkellipsoide) of the alum-slates it occurs in "long, lath-shaped, 

 irregularly- bounded individuals showing twin striation." The extinc- 



(1) Q.J.G.S., Vol. XXXII. (1876), p. 411. Professor ROSENBUSCH throws doubt on this 

 identification of felspar in a contact-rock by Mr. ALLPOET, Mik. Phys. Band II. (1886), 

 p. 53. The present writer has examined Mr. ALLPOET'S specimens, and has no doubt as to 

 the existence of felpar iu this rock, 



