390 
POPULAR SCIENCE REYIEW. 
different from that used for heights or vertical distances. In 
other words, a measure of distances in length being assumed to 
bring so much of a section as is desired within the range of a 
certain space, a measure of distances in height is assumed quite 
independently, which shall also exhibit all that is desired. 
Thus, if it were desired to show a section across five miles of 
country within the breadth of an ordinary page of letter-press, 
the scale must be about an inch to one and a half mile (nearly 
one to a hundred thousand). Now, under ordinary circum- 
stances, the total difference of height in such a section on a tole- 
rably level country would not exceed five hundred feet, and on 
the scale just mentioned this height would amount to only one- 
sixteenth of an inch, far too small to admit of the representation 
of any geological facts. To alter the section so that these facts 
may appear, there must be a different scale for heights and 
distances, and the scale will be more or less disproportioned 
according to the difference of height that has to be represented. 
But in this there results a distortion which it requires much 
study to overcome the effect of. The scale of heights in the 
case represented in the diagrams 4, 5, must be at least ten 
times as great as that of distances to admit of any adequate 
representation ; and the effect of this is shown in some measure 
in fig. 5 of the Plate, where the section in fig. 4 is repeated 
to a scale of which the heights are five times greater in pro- 
portion than the distances. It can hardly be necessary to do 
more than point out to the student how complete a distor- 
tion this change makes, but the only alternative is to increase 
the length of the section. Owing to the necessity for distor- 
tion, many geological sections are little more than diagrams, 
and are very difficult for the student to understand correctly. 
The distortion when admitted should be carefully borne in 
mind, and it is hardly possible to base a serious conclusion on 
.a section without reducing it to an equal scale of heights and 
distances. 
Everything connected with the mechanical disturbances of 
beds and their direct results may be shown conveniently and well 
in geological sections. Anticlinal and synclinal axes (known 
under various names), curving and distortions of strata, inter- 
ruptions of strata, faults and dislocations, veins and dykes, false 
stratification — all these and many phenomena of stratification, 
including the peculiarities of structure of rocks, are thus de- 
lineated, and in case of need accurately expressed. 
• The horizontal and vertical sections published in illustration 
of the Greological Survey Map of England will be found very 
useful to the student as expressing groups of facts, but they 
must not be taken as superseding personal observation even in 
places very near the actual line of country represented. Nor 
