428 D. MACKINTOSH ON THE ERRATIC BLOCKS OF 
or suddenly increasing temperature, or both; and land-ice as well 
as floating ice would be vulnerable to both these kinds of ob- 
structions. Rising ground would furnish the most permanent and 
fixed obstruction to the transportation of boulders; but in those 
places where the frontal termination of boulder dispersions is not 
coincident with rising ground, contermineity of granite and felstone 
dispersions could only have resulted from close proximity to a per- 
sistent current of water (or air?) sufficiently warm to melt the 
boulder-laden ice. In this way the sharply defined frontal boundary 
of the Criffel-granite and Lake-district-felstone boulders at Bush- 
bury (near Wolverhampton) may have been made to coincide; for 
there the rise in the ground is not sufficient to have furnished an 
obstruction (considering the height of the ground the boulders must 
previously have surmounted). Neither is the rising ground between 
Wolverhampton and Bridgenorth sufficiently continuous to haye ob- 
structed the further transportation of boulders. A warm current 
in front, however, would not cause lateral contermineity ; and we 
accordingly find that the Bushbury granite and felstone boulders 
are not laterally conterminous. As regards the granite and felstone 
of the Lake-district, their proximity to each other in situ renders it 
likely that in many instances they may have been launched at the same 
time and kept company until they encountered one of the local 
obstructions already noticed, im which case they may have been 
conterminously precipitated both frontally and laterally, as in the 
ease of the Burton concentration (see III. § 7). 
6. Overshot Boulder-loads—So far as has yet been discovered, 
there would appear to be different boulder-groups, like fallen loads, 
which occur along the same lines of dispersion too regularly to admit 
of the supposition that one deviated from its straight-onward course 
so as to come in front of or behind another. In these cases it may 
be allowable to suppose that one of the boulder-loads came to rest 
before the other passed over its site, and was precipitated a greater 
or less distance in advance of it, so as to prevent a commingling of 
the two boulder-loads. These may be called overshot boulder-loads ; 
but the term may likewise be applied to loads which have passed 
over the sites of previously precipitated loads obliquely, and not 
along the same lines of dispersion. Of both these kinds of loads 
instances will be noticed in the sequel. Some of them are large 
enough to be called intermediate concentrations. 
7. Changes in the Positions of Boulder-laden Currents accompanying 
Changes in Surface-level and Temperature.—Ilt seems very obvious 
that currents must have changed their positions, and (with the ex- 
ception of a general southerly trend in the northern drift-area) their 
directions, as the land sank and rose; and (as a consequence) the 
depth of the sea varied. This must likewise have been the case as 
the temperature of the water changed, not only in particular spots, 
but over the whole of the boulder-strewn area. Facts mentioned in 
the sequel would seem to show that during the first stages of the 
submergence the cold, and consequent extent of floating ice, increased 
southwards, and that during the last stages (while the land was 
