ROCKS 77 
spots or concretionary knots which in some cases seem to be incipient 
stages in the development of such minerals as andalusite and cordierite ; 
Andalusite-, Chiastolite-slates, slates marked by the presence of these 
minerals in less or greater abundance. Spotted slates and andalusite- 
slates, etc., may be little altered otherwise, or they may contain much 
mica, and so gradually pass into andalusite-mica-schist. Phyllite is 
more crystalline than clay-slate. It isa schistose clay-rock, the cleavage- 
planes of which are lustrous with white mica, and frequently finely 
wrinkled. It is a passage-rock between clay-slate and mica-schist. 
Many of the hard rocks included under the term //orzfels are meta- 
morphosed argillaceous rocks. [Some, however, are altered igneous 
rocks, while others appear to have been originally impure limestones 
or dolomites (calc-stlicate hornfels).| 
Mica-schist is a crystalline schistose aggregate of mica 
and quartz, varying in texture from fine-grained to coarsely 
crystalline. Of the two constituent minerals, sometimes the 
one and sometimes the other predominates, or they may be 
present in approximately equal proportions. The quartz 
occurs as granules or granular aggregates, and extends in 
lenticular layers, thinning- and swelling-out more or less 
suddenly: sometimes it assumes the form of irregular 
nodule-like bodies, around which the foliated mica bends. 
Accessory minerals are of common occurrence, such as garnet, 
specular iron (iron-mica), magnetite, rutile, schorl, etc. 
Gneiss is a schistose aggregate of quartz, felspar, and 
miles (tuscovite, biotite—either or both). The texture is 
very variable. Sometimes the rock is fine-grained and the 
i/tasate thin and even (Plate X XII. 2): in other cases the 
folia may be so thick, irregular, or indistinct that in hand- 
specimens the schistose nature of the rock may not be 
apparent. The proportion of the several component minerals 
also varies indefinitely—one or other often greatly pre- 
dominating. | Accessory minerals are common, such as 
garnet, apatite, iron-ores, schorl, rutile, etc. 
Many varieties are recognised :—as, Horndblende-gneiss, with horn- 
blende and often mica; Aug7te-gnezss, with a pale green pyroxene and 
little or no mica; Profogine-gneiss, with a hydrous mica in place of 
mica; Graphite-gnetss, with graphite replacing mica in whole or 
part; Chlorite-gneiss, with chlorite instead of mica; Grantte-gnetss, 
with indistinct foliation ; Augen-gneiss (Eye-gneiss) with large eye-like 
kernels (fhacozds) of quartz or orthoclase (see Plate XXIII.). 
The gneisses in which a foliated structure is well developed are 
