216 JOURNAL OF THE PLYMOUTH INSTITUTION, 
(primitive) not in contact with the granite, greywacke formation up 
to the limestone, rocks of the shore, and rocks of the Eddystone. 
No better course was open to him; but we can choose what may 
be called a structural-historical arrangement, and treat together 
all the rocks of similar origin and character under the following 
heads: Sedimentary, contemporary igneous, intrusive igneous, 
granites and elvans, altered rocks. But each head requires sub- 
division. 
SEDIMENTARY ROCKS. 
The stratified rocks of the district need not detain us long. 
They afford good examples of each of the great water-formed 
divisions—argillaceous, arenaceous, and calcareous, and that in 
some variety. 
Slates.—The larger part of the area is occupied by clay-slates, 
more or less strongly marked by cleavage ; really hardened clay, 
an indurated hydrous silicate of alumina. The varieties are 
very numerous. There are the hard, blue, finely-cleaved slates, 
well adapted for roofing purposes, of Cann. Wood, and other 
localities ; the loose, broken, subsoil slates, commonly known as 
“‘shillet”; soft drab or olive slates, which split into flakes of little 
tenacity ; soft drab splintery slates; compact claret and green 
slates, well seen in the railway cutting near Mutley, which have 
all the appearance of a definite lithological horizon ; and the more 
massive, brown, thickly-jointed, distinctly-charactered slate, which 
occupies a wide range to the northward, among the lowest of 
our local rocks. These slates range in hardness from varieties 
that ring under the hammer to others that will crumble between 
the fingers; in colour, from a dark-leaden blue or ruddy-purple 
to brown, drab, olive, pale-green, grey, and ochreous-yellow ; in 
texture, from finely-grained roofing material to a coarse sandy- 
shale, which marks at times the passage into sandstone; in 
structure, from the highly cleavable slate of Cann Quarry, to the 
junk-like, massive, rhombohedral-fracturing strata seen by the side 
of the Tavistock railway, near Bickleigh, Yelverton, or Horra- 
bridge, and to the splintery fossiliferous band of Staddiscombe ; 
in composition, from rocks of the normal silico-aluminous type to 
others largely charged with iron oxide, or, as frequently happens 
in the case of those immediately contiguous to the limestone, with 
calcareous matter. These, however, are rather to be regarded as 
