Hints for the Formation of 
202 
CHAP. XI. 
Observations to be made on Fissures . 
1. Their form, dimensions, breadth, extent, and di- 
rection. 
2. Their situation ; measure of their inclination ; di- 
rection of that inclination in regard to the cardinal points, 
and in regard to the adjacent mountain and valleys. 
4. To observe, above all, the direction of fissures in 
regard to that of the planes of the strata ; because fis* 
sures, as is presumed, being produced, in general, by the 
earth sinking down, and this sinking down being the ef- 
fect of pressure, fissures have been originally vertical or 
nearly so ; and, on the other hand, because the sttata 
having originally been horizontal or nearly so, the situa- 
tion of fissures, in regard to the strata, and the direction 
of both in regard to the horizon, may give some idea of 
the situation which the strata had when the fissures were 
formed, and even of the changes of situation which the 
mountain afterwards experienced. 
Thus, fissures perpendicular to the planes of the stra- 
ta, indicate that these fissures were formed when the 
mountain was still in its primitive situation ; and if they 
are also perpendicular to the horizon, it proves that the 
mountain is still in the same situation : but if fissures 
perpendicular to the strata are inclined to the horizon, 
we may conclude that the mountain has changed its si- 
tuation since these fissures were formed.* 
5. When the fissures are filled with matter different 
from the body of the mountain, that matter is called a 
vein. 
6. Lastly, one must examine, in both sides of the same 
* An explanation and application of these principles may be seen in my 
Travels, § 1048, 49, 50, and 1218= 
