388 



may sometimes be recognized as an accessory constituent. The main mass 

 of the granite contains black as well as white mica. 



Mr. WORTH ^ has described the contact metamorphism produced by the 

 Dartmoor granite. He says : " The alteration in the adjacent belt of slates, 

 as a rule, proceeds so systematically that it would be quite possible with 

 a little trouble to locate, within a very few yards, the position of a stray 

 specimen from any given area. There are, however, differences in the 

 changes in different localities, apparently due to two causes first, the 

 original character of the rock altered ; secondly, the relation borne to 

 it by the granite in bulk and position." The common type of meta- 

 morphism along the southern border of the granite area is the conversion 

 of the ordinary clay-slates into spotted-slates "by the development of 

 nodules or crystals, more or less imperfect, of andalusite or its variety 

 chiastolite." As the granite is approached the rocks become more and 

 more micaceous and finally pass into micaceous schists tfr into a massive 

 rock which has lost all distinctive traces of its original fissile structure 

 (hornfels). Chiastolite slate occurs at Ivybridge ; andalusite slate at 

 Cornwood. 



Leicestershire. An interesting case of contact metamorphism may 

 be observed in Brazil Wood near Mount Sorrel. It has been described by 

 Mr. ALLPORT. (2) The granite which has produced the metamorphism is 

 hornblendic and, near the junction, appears somewhat porphyritic in 

 consequence of the development of well-formed oligoclase - crystals. 

 Under the microscope these crystals show a well-marked zonal banding 

 and the usual polysynthetic twinning. The ground-mass is composed of 

 quartz, turbid felspar, dark mica, magnetite and a little hornblende. 

 Garnet occurs as an accessory constituent. The junction with the altered 

 rock is sometimes sharp and sometimes ill-defined, as if the constituents 

 of the granite had impregnated the surrounding rock for a short 

 distance. The altered rock is exposed only in one small quarry so 

 that no information can be obtained as to the progressive changes as 

 the granite is approached. The common variety of altered rock is a 

 highly micaceous hornfels. This variety is, however, sometimes banded 

 with a compact rock in which mica is rare. It is evident that the 

 original sediments varied in character, and that the different beds have 

 been differently affected. The micaceous rock is essentially composed 

 of two micas, quartz and magnetite. In some specimens the dark mica 

 has been changed to chlorite. The individual constituents are arranged 

 without order in the rock. The compact rock is mainly composed of 

 an aggregate of water-clear grains having the refraction and double- 

 refraction of quartz or felspar. It also contains magnetite, a few flakes 

 of mica and a considerable number of minute rounded grains of 

 colourless garnet. Red garnets occur in well-formed dodecahedral 

 crystals in certain varieties of the rock. They are almost always 

 associated with quartz grains which sometimes form a kind of irregular 



(1) Trans. Plymouth Inst., &c., 1886. Vol. I., pp. 19-44. 



(2) G.M. Decade II. Vol. VI. (1879), p. 481. 



