516 Alexander Scott — The Grawfordjohn Essexite. 



closely resembles the caraptonite (column 5, table iii), which 

 Brdgger regards as a complementary differentiate of the essexitic 

 magma of the Gran district. 1 



The Contact Metamorphosed Rocks. — In both quarries an interesting 

 set of altered rocks is found in contact with the margin of the 

 intrusion. The sediments range from greywackes and grits, of 

 varying degree of coarseness, to fine-grained mudstones and shales. 

 The grits and greywackes, which consist of rounded grains of quartz 

 and felspar, together with rock-fragments, are not much altered, the 

 alteration being more or less confined to the matrix, and being there- 

 fore more noticeable in those rocks with a comparatively small 

 proportion of granular material. In the more altered rocks the 

 grains seem to have undergone some corrosion, while the matrix has 

 been completely recrystallized. The chief 'new-formed' minerals 

 comprise ilmenite, in small eumorphic crystals, which are often brown 

 in colour and translucent, and small greenish prisms with a good 

 cleavage, which seem to be muscovite altering to chlorite. The 

 remainder of the matrix, which is stained by iron oxide, is felsitic in 

 structure and appears to be an aggregate of felspar. Some of the 

 rocks contain microlites of felspar, while others have bunches of 

 hair-like crystals, which are probably rutile, and sporadic flakes of 

 strongly pleochroic biotite. 



The shales, however, have undergone a much greater amount of 

 metamorphism. In one rock which occurs in the large quarry 

 numerous lath-shaped crystals can be seen in the hand-specimen, 

 while within the space of 3 inches there is a passage to a thoroughly 

 aphanitic, compact rock with no recognizable minerals. Under the 

 microscope the former rock shows rather a remarkable structure, as 

 it consists entirely of interlocking, irregularly shaped crystals of 

 colourless cordierite, which enclose all the other constituents. The 

 outlines of the cordierite grains are not usually visible in ordinary 

 light, but between crossed nicols the crystals appear as irregular 

 prisms. Simple twinning is very common, but multiple and complex 

 twins are rare. Of the enclosed minerals, muscovite, often altered to 

 chlorite, is the most abundant. Biotite and ilmenite are also present 

 and occasionally predominate in certain bands, the determining factor 

 being the composition of the particular band. This cordierite-hornfels 

 has a close resemblance to the cordierite schist described by Harker 

 from the Skiddaw district. 2 Not only is the cordierite quite fresh 

 and unaltered in both instances, but it also constitutes a coarse- 

 grained groundmass in which the remaining, much smaller, crystals 

 are set. As the compact rock is approached the grain-size of the 

 cordierite diminishes and the crystal boundaries become indistinct, so 

 that finally the rock consists of small crystals of muscovite with 

 subordinate biotite and rutile in a fine-grained matrix of cordierite. 



Another type of altered argillaceous rock occurs in the small quarry 

 and also further up on the hillside. Under the microscope, the rock 

 is seen to consist of a great abundance of minute crystals of muscovite 

 with subsidiary ilmenite in a groundmass, which is partly composed 



1 W. C. Brogger, loc. cit. 



2 A. Harker, The Naturalist, pp. 121-3, 1906. 



