accumulated evidence of interglacial periods, and assuredly the
establishment of such periods is of paramount importance for understanding
all the later changes of the earth's surface. Reading your book has
brought vividly before my mind the state of knowledge, or rather ignorance,
half a century ago, when all superficial matter was classed as diluvium,
and not considered worthy of the attention of a geologist. If you can
spare the time (though I ask out of mere idle curiosity) I should like to
hear what you think of Mr. Mackintosh's paper, illustrated by a little map
with lines showing the courses or sources of the erratic boulders over the
midland counties of England. (514/2. "Results of a Systematic Survey, in
1878, of the Directions and Limits of Dispersion, Mode of Occurrence, and
Relation to Drift-Deposits of the Erratic Blocks or Boulders of the West of
England and East of Wales, including a Revision of Many Years' Previous
Observations," D. Mackintosh, "Quart. Journ. Geol. Soc." Volume XXXV., page
425, 1879.) It is a little suspicious their ending rather abruptly near
Wolverhampton, yet I must think that they were transported by floating ice.
Fifty years ago I knew Shropshire well, and cannot remember anything like
till, but abundance of gravel and sand beds, with recent marine shells. A
great boulder (514/3. Mackintosh alludes (loc. cit., page 442) to felstone
boulders around Ashley Heath, the highest ground between the Pennine and
Welsh Hills north of the Wrekin; also to a boulder on the summit of the
eminence (774 feet above sea-level), "probably the same as that noticed
many years ago by Mr. Darwin." In a later paper, "On the Correlation of
the Drift-Deposits of the North-West of England with those of the Midland
and Eastern Counties" ("Quart. Journ. Geol. Soc." Volume XXXVI., page 178,
1880) Mackintosh mentions a letter received from Darwin, "who was the first
to elucidate the boulder-transporting agency of floating ice," containing
an account of the great Ashley Heath boulder, which he was the first to
discover and expose,...so as to find that the block rested on fragments of
New Red Sandstone, one of which was split into two and deeply scored...The