CHARACTERISTICS OF GLACIAL DRIFT. 
167 
Arctic species, and frequently in a fragmentary state. The 
bulk of the till has usually been derived from the grinding 
down into mud of rocks in the immediate neighborhood, so 
that it is red in a region of Red Sandstone, as in Strathmoi*e 
in Forfarshire; gray or black in a district of coal and bitu¬ 
minous shale, as around Edinburgh ; and white in a chalk 
country, as in parts of Norfolk and Denmark. The stony 
fragments dispersed irregularly through the till usually be¬ 
long, especially in mountainous countries, to rocks found in 
some part of the same hydrographical basin; but there are 
regions where the whole of the boulder clay has come from 
a distance, and huge blocks, or erratics,” as they have been 
called, many feet in diameter, have not unfrequently travelled 
hundreds of miles from their point of departure, or from the 
parent rocks from which they have evidently been detached. 
These are commonly angular, and have often one or more of 
their sides polished and furrowed. 
The rock on which the boulder formation reposes, if it con¬ 
sists of granite, gneiss, marble, or other hard stone, capable 
of permanently retaining any superficial markings which may 
have been imprinted upon it, is usually smoothed or polish¬ 
ed, like the erratics above described, and exhibits parallel 
striae and furrows having a determinate direction. This di- 
rection, both in Europe and North America, agrees generally 
in a marked manner with the course taken by the erratic 
blocks in the same district. The boulder clay, when it Avas 
first studied, seemed in many of its characters so singular and 
anomalous, that geologists despaired of ever being able to 
interpret the phenomena by reference to causes now in ac¬ 
tion. In those exceptional cases where marine shells of the 
same date as the boulder clay were found, nearly all of them 
were recognized as living species—a fact conspiring with the 
superficial position of the drift to indicate a comparatively 
modern origin. 
The term ‘‘diluvium” was for a time the most popular 
name of the boulder formation, because it was referred by 
many to the deluge of Noah, while others retained the name 
as expressive of their opinion that a series of diluvial waves 
raised by hurricanes and storms, or by earthquakes, or by the 
sudden upheaval of land from the bed of the sea, had swept 
over the continents, carrying with them vast masses of mud 
and heavy stones, and forcing these stones over rocky sur¬ 
faces so as to polish and imprint upon them long furrows and 
striae. But geologists Avere not Icfng in seeing that the.boul¬ 
der formation was characteristic of high latitudes, and that 
on the Avhole the size and number of erratic blocks increaLCS 
