HOW TO MAKE A GEOLOGICAL MAP. 
33 
building and paving, for millstones, &c. ; sands and gravels, 
for making paths and mending roads ; clays and loams, for 
the manufacture of tiles and bricks ; slates, for roofing pur- 
poses and for school-boys to draw upon ; marbles, for the 
sculptor and for ornamental purposes ; granites, for building 
and for road-metal. 
Of course the economic value of each rock cannot be made 
out from the map alone ; this is a point on which reference 
must be made to a published memoir or explanation of the 
map, upon which alone the boundaries and extent of the rocks 
can be depicted. 
The relation between health and geology is also a point 
which has in recent years received a good deal of attention ; and 
maps have been published and memoirs written to show the 
relations between certain forms of disease and geological struc- 
ture — even between geology and lunatics ! It is well known, 
indeed, that a gravelly, sandy, or chalky soil is more healthy 
than a clay foundation, because the former are pervious to 
water, and the latter is impervious. On the former there is 
less consumption than on the latter, as Mr. Whitaker and Dr. 
Buchanan have clearly demonstrated : the artificial removal of 
sub-soil water has, however, largely decreased it. Again, the 
water-supply is a most important subject, for in some small 
country villages and towns the inhabitants suffer very much 
from this cause. Situated perhaps on elevated ground, with a 
good porous soil, they yet suffer because of the disgraceful state 
of the drainage, the wells being shallow and the sewage, even 
the churchyards, draining into them. The cause of teetotalism 
will not find many admirers when it is known that women and 
children suffer most from drinking impure water, while the 
men who take their beer are less subject to disease. 
Enough has been said to show the many practical uses of 
geology, and the importance of geological maps. To the agri- 
culturist their value is of an indirect character ; for although 
the soils are not laid down on the maps, yet in almost every 
case they bear a direct relation to the sub-soil beneath, being 
generally to a great extent formed out of it. 
We need hardly dwell upon the interest imparted to a tour 
or journey from an acquaintance with geology, it has been so 
frequently remarked upon. Undoubtedly, a good geological 
map is to the traveller the readiest, if not the best, mode of 
obtaining information. It furnishes a sort of index to the 
geology of a country, and is for most purposes to be preferred 
to any written description or guide, though if possible both 
should be taken together. In the British isles, as we have 
pointed out, we are well provided with geological maps, and 
there are many excellent ones of the greater part of Europe, 
VOL. XII. — NO. XLVI. D 
