HOW TO MAKE A GEOLOGICAL SECTION. 
391 
must they be supposed to be in all cases accurate. Like other 
observations made for an extensive survey they involve large 
generalisations, and are useful almost more as suggestions than 
as completed works. Of the two the vertical are the more 
valuable, as they are strictly accurate. The horizontal are often 
little more than expressions in a pictorial form of the facts 
already stated on the geological map. 
The geologist often makes use of sections very roughly drawn, 
and without any pretension to accuracy, which serve as diagram- 
matic representations of facts either striking in themselves or 
important to be noticed. Such rough sections must not be 
mistaken for true geological sections. They are constructed 
without much reference to scale or proportion, and they are 
therefore only valuable for the special purpose for which they 
are introduced. They may, however, be very useful and in- 
structive, and are entered in the note book on the field or 
prepared for the lecture-room without hesitation. They are in 
fact sketches. They differ from artists’ sketches in not repre- 
senting the general features of a landscape, but the geological 
features of a cliff, a quarry, or a cutting. Grrand phenomena 
of disturbance and of the intrusion of igneous rocks are thus 
conveniently marked. Of this nature also are the diagram- 
matic sections that include a considerable distance of country ; 
and they are necessary for this purpose, as it is practically im- 
possible to give accurate sections extending for more than a few 
miles. In other cases the great and characteristic features of a 
small district must be magnified, and those that are monotonous 
reduced. In any case the result, though very useful as giving 
a general idea of the structure of a continent, an island, or a 
small district, has no pretensions to strict accuracy. In fig. 6 
a section of this kind is given. It is one of a kind whose 
object is to assist the memory of the student in reference to 
the main divisions of rocks. The particular diagram refers to 
the rocks as developed in England, and gives in a rough way 
an idea of what would be seen in crossing our island from 
Suffolk to Wales. In such a trip the various groups of rocks 
would be crossed in regular order, beginning with the Tertiary 
Crag in the East, and ending with the oldest known stratified 
rocks on the Welsh coast. The distance is 250 miles or there- 
abouts, the extreme height about 3,000 feet. The scale of 
heights to distances is therefore about one-sixtieth; in other 
words, an inch of the section represents about 60 miles in length 
and about 5,000 feet in height. But there is no attempt at an 
actual representation of relative heights or of the proportionate 
development or relative importance of any of the groups. The 
total thickness of strata represented is upwards of 30,000 feet, 
but in all probability, if a deep shaft were sunk in Suffolk, the 
