58. REPORT—1858. 
size, of perfect clearness and accuracy in the laying down the most complex 
localities, and those in which the shocks are most numerous. This has been 
reproduced to a much reduced scale (Plate XI.), to accompany the present 
Report ; but although executed with much skill and care, by the lithographer 
and engraver, I find with regret that its small size has rendered_a perfectly 
accurate transcript of the original impracticable, and that a very imperfect 
notion of the latter is conveyed by the reduced map. 
Strictly, the limits of every earthquake are completely indeterminate ; and 
were our globe perfectly solid, homogeneous, and elastic, no limits but its 
own could be assigned to any shock from whatever centre originating. The 
practical limit (so to speak) is, however, where the moyement has become 
insensible without instrumental aid ; for such have been all the observations 
dealt with in our Catalogue. This frequently embraces enormous surface- 
areas; but these seldom, perhaps nowhere, are symmetrically posited round 
the centres, or presumed centres, of disturbance. 
We are not concerned here with any of the smaller or local circumstances 
that modify, in different radii traced from any seismic centre, the effects, 
and the directions and distances, to which they are sensibly transferred, but 
merely with some of the greater and constant conditions (for the same region) 
in which some of the great natural features of the earth’s surface perma- 
nently modify or limit the transference and area of transfer of earthquake- 
waves transmitted from adjacent centres. Thus, along the whole chain of 
the South American Andes, the propagation of shock is greatly more 
towards the west than to the eastward,—the highest crests and intermediate 
valleys forming a rude sort of limit, beyond which, to the eastward and into 
the heart of the table-land of the continent, shocks felt with destructive 
effect down to the shores of the Pacific are propagated with greatly di- 
minished force, or rather are so felt upon the surface, 
Again, to take another large example, the Northern Indian earthquakes, 
whose origin is in Nepaul and along the central Himalayan axis, are pro- 
pagated southwards and westwards into the great plain of India, far more 
than. northwards into the enormous mass of table-land of Central Asia. 
Weare at this moment not concerned with the causes of this, but simply 
with the fact, that in these examples, and in several analogous instances, it 
is a matter of observation that certain great natural features of the earth’s 
surface and material, do modify the forms of the surface-areas shaken, 
and render them unsymmetrical, shortening the radii in one direction, 
lengthening them in another; so that the area, which in a more homo- 
geneous mass would approach a circular or elliptic form, tends to an elon- — 
gated, linear, or irregular outline. 
In laying down, then, the forms and sensible area of shock of each earth- 
quake catalogued (and often necessarily, from the imperfect data alone 
afforded), the following rules were adhered to :— 
1°. Wken the form and sensible limits of the shaken area were ascertain- 
able from the narratives, they were adopted. 
2°. When these were wanting, as in the great mass of cases recorded, then, — 
as respects form, the physical, geological, or other conditions of 
each area, known to modify the distant propagation of shock, were 
attended to. 
3°. As respects sensible area, when this could not be ascertained for 
any one diameter of the shaken area, from the narratives, certain 
arbitrary conventional rules (founded upon a natural basis, however) 
were resorted to. 
