16 
instances to trace the development of one set of animals from the 
preceding set. 
Now this regularity prevails with scarcely any break through the 
whole series of rocks from the Archean or oldest known beds to those 
called Tertiary. In all cases we can tell the relative age, the mode 
of formation and deposition, and the characters of the various kinds 
of life then prevalent. Then suddenly this regularity seems to 
end, aS we come upon a number of scattered formations to which 
the regular laws apparently do not apply, and for a long time the 
geologists were at a loss to account for this. We rarely find any 
animal or plant remains in these deposits; they are not arranged 
in layers or strata; they seem scattered promiscuously in various 
localities, and they repose upon rocks of all ages. All that we can 
say of them at first sight is, that they are more recent than 
all the rest. At first their apparently confused and disorderly 
arrangement (if indeed such a term is applicable at all), was taken 
as proof and consequence of a confused an@ disorderly period in the 
earth’s history. It was taken for granted that they represented a 
‘‘ oreat gap separating the present from the past. Some mighty 
convulsion of nature was thought to have marked the close of the 
geological ages, and to have immediately preceded the advent of 
man, and the introduction of the plants and animals with which 
he is associated.”’ But, says James Geikie, ‘‘ the study of these 
deposits has unfolded a deeply interesting and almost romantic 
history. We are introduced to scenes that are in strangest 
contrast to what now meets the eye in these latitudes; geological 
and physical changes of the most stupendous character pass before 
us; we see our islands and northern Europe at one time enveloped 
in snow and ice, at another time well wooded and inhabited by 
rude tribes of men and savage animals; now the British islands 
are united to the continent,—again, the sea prevails, and a large 
part of Britain is overwhelmed beneath the waters of the ocean, 
across which ere long float rafts and bergs of ice. ‘To these suc- 
ceed vast confluent glaciers which overflow a Jarge portion of the 
British area, and usurp the bed of the sea. Yet again, we beho!d 
the great icefields vanish away and Britain once more becoming 
continental and re-peopled. Finally, we follow the working of 
those physical influences, by which at last the present order of 
things is brought about.’ In fact, we may say that what seemed 
a puzzle at first, has now become a key to at least one chapter in 
ancient history. 
Let us see then, what are the characters of these deposits from 
which such a romantic history has been extracted? There are 
three:—Their apparently disorderly distribution, their varied 
composition, the stones and blocks of all sizes contained in them 
