on the Great Derbyshire Fault. 125' 
and the map which accompanies it, to be considered as 
presenting only a rapid sketch of the more important 
characters, without any attempt at minute accuracy of de- 
tail, which would be more likely to fatigue attention than 
to excite or gratify general curiosity.” 
There are three distinct modes of forming a geological 
map: the one proposes to trace every stratum as it rises to 
the surface, and to delineate its boundaries and termination. 
This can only be applied to small districts or estates. The 
second traces the situation, the different orders of rock, and 
the more remafkable strata or mineral repositories: this may 
be applied to larger districts. The third mode delineates 
the situation of the different classes of rock, and presents” 
only the leading features of the mineral geography, without 
attempting minute accuracy of detail : this is suited to con- 
vey, in a concise form, a view of the most important geo- ~ 
logical characters of a kingdom. Such is the Map of the 
United States, given by M. Maclure in the Journal de Phy- 
sigue ; and this is what I have attempted in the Map of 
England to which Mr. Farey alludes. The upper cal- 
careous strata, comprising calcareous sandstones and chalk, 
occupy the eastern side of England; the western termina- 
tion of these strata is traced in a waving line from Dor- 
setshire to near Scarborough. In some situations the cal- 
careous strata may extend a few miles beyond the line, or 
terminate a few miles east of it: but such minuteness of 
delineation was unattainable on the scale in which it is 
given. The next great division includes the lower secon- 
dary strata, which contain in various parts ironstone, rock- 
salt, and coal, but without any delineation of the particular 
coal-fields, the most remarkable of them being described 
in the work. These strata are terminated by the alpine 
district on the western side of England, containing metallic 
ores. 
Mr. Farey observes, that the inhabitants of Exeter would 
be obliged to me to find the characteristic minerals of the 
middle district, ‘*iron-stone, rock-salt, and coal.” But 
Mr. F. can scarcely be so unacquainted with coal strata, as 
not to know that they exist only in detached patches in the 
secondary districts to which coal is peculiar, and many ex- 
tensive districts of Jower secondary strata contain no beds 
of workable coal. But it is not the less true that such strata 
are the proper repositories of coal. That these strata ex- 
tend into Devonshire, is well known. Vide Dr. Berger, 
Mr. Townsend, and others. 
Any one who reads Mr. Farey’s paper might wa 
that 
