183 
the killas and shillot beds of Devonshire and Cornwall; and he 
therefore considers himself fully entitled, on lithological evidence, to 
assign to these rocks that place in the general series which they 
occupy in the present map. The evidence to be derived from Pa- 
leontology, though incomplete, he is of opinion, also points out to 
the same conclusion. 
The <<‘ earthy sooty limestone’? which rests conformably on these 
beds near Barnstaple, Holcomb Rogus and West Leigh, with a south- 
erly dip, and rises again with a contrary dip at Launceston and South 
Petherwin, Mr. Greenough considers on zoological evidence to be the 
representative of the true mountain limestone. Under these circum- 
stances, he thinks it highly probable that the culm limestone of 
Barnstaple, &c., occupies the same place in the general series as that 
which in other countries forms the floor of our coal-field; and con- 
sequently that the superincumbent shales, marked with vegetable im- 
pressions, are legitimate members of the great carboniferous system. 
If further evidence on this subject be required, Mr. Greenough 
says, there is still in reserve another argument with which Mr. 
Murchison has kindly supplied him, derived from his recent re- 
searches in Germany,—an argument of analogy. 
The transverse sections in descending order from the well-known 
coal-field of Westphalia (notably in the zone between Hagen and 
Iserlohn) exhibit clearly a descending order from the productive coal 
strata, through beds representing the English millstone grit, and 
then through various schists and sandstones, with minute plants re- 
sembling those of the culm-measures of Devon; next through bands 
of flag-limestone, undistinguishable from the black limestones of 
Barnstaple, and, like them, containing Posidonia and Goniatites, and 
associated with bands of flinty slate. (Kiesel- schiefer). Passing to 
still lower strata of schists and psammites, other calcareous courses 
appear, including the great limestone of ale tract, which contains 
the same fossils as the limestone of South Devon ; and finally, these 
are underlaid by greywacke and subordinate courses of limestone, — 
which rise into the higher mountains of the region, and contain 
Homalonoti, Pterinez, and other shells characteristic of the upper 
members of the Silurian System. 
In the third place, Mr. Greenough explains the principles by which 
he was guided in the choice and distribution of colours. 
In the choice of pigments he sought the most transparent, the most 
distinct ; those least subject to fade by exposure to air, light, or 
damp ; those which are most soluble, and those of which the ingre- 
dients will not act chemically one upon another. 
In the distribution of pigments, he endeavoured to accommodate 
the colour of the pigment to that of the substance represented ; to 
apply to substances mineralogically similar, similar tints; to sub- 
stances mineralogically dissimilar, dissimilar tints; to place in juxta- 
position those colours only which would either harmonize or contrast, 
as the occasion might require; to confine the opake colours to those 
parts of the map which are least charged with engraving ; to reserve 
the most forcible colours for the smaller spaces; to denote marked 
