Revieiv of Bece?it Geological Literature. 343 
fore a possible clue to the composition of the deeper seated layers of the 
earth . 
Professor Prestwich traces the history of the glacial period as follows: 
First, extreme glaciation, an ice-sheet spread upon the greater part of the 
British Isles, and the formation of the lower boulder-clay or till; second, 
a depression of 1,500 to 2,000 feet in central England, Wales and Ire- 
land, attended with warm marine currents and a more southern marine 
fauna; and third, re-elevation, when the warmer currents were diverted or 
stayed, colder conditions resumed, and Arctic mollusca returned for a 
time to the coasts of Scotland. The interglacial deposits and vicissitudes 
of climat'^ shown by them are attributed to "changes in the physiography 
of Europe,"" rather than to "the cosmical causes to which the Glacial 
Epoch, as a whole, was there can be little doubt, due." The author, how- 
ever, dissents from Dr. Croll's astronomical theory, which would place 
the glacial period between 240,000 and 80,000 years ago. After comparing 
the present glaciers of Greenland, which are found to flow 30 to 50 feet a 
day throughout the year, with the glaciation of the great drift-covered re- 
gions during the Ice Age, he says: -'My own opinion, based on the facts 
here named, is — that the time required for the formation and duration of 
the great ice-sheets of Europe and America (the Glacial Period) need not, 
after making all allowances, liave extended beyond 15,000 to 25,000 yearsi 
instead of the 160,000 or more which have been claimed." 
The time since the melting away of the ice-sheet is estimated to be 
8,000 to 10,000 years or less, making the antiquity of palaeolithic man no 
greater than about 20,000 to 30,000 years. "This view of the question," as 
the author further remarks, "also brings the geological and anthro- 
pological data into close relationship. Pala;olithic Man in North-west- 
ern Europe disappeared with the valley gravels. With the alluvial 
and peat beds Neolithic Man appeared after an unascertained, but clear- 
ly not very long interval geologically speaking. In Europe we are 
unable to carry back his presence beyond a period of from 2,000 to 
3,000 years B. C. But already in Egypt, and in parts of Asia, it is 
proved that civilized communities and large States flourished before 
4,000 B. C. Civilized Man must therefore have had a far higher 
antiquity in those countries, and probably in Southern Asia, than 
these 4,000 or 5,000 years; so that comparing Europe and Asia, it 
is possible that the two periods may have over-lapped, and that while 
Man had advanced and flourished in a civilized state in the East he may 
here in the West have been in one of his later Paheolithic stages." 
Mock-forming mineraU. By Fr.vnk Pidtley, F. G. S. With 126 illustra- 
tions, 252 pp., London, 18S8. This is a compact, useful text-book for use 
in the laboratory and classroom; one of the few in the English language. 
It descril>es the essentials of a petrographical microscope, and their use 
also the processes of making thin sections of crystalline rocks and the ac- 
cessory appliances. It is very e>:p!icit and satisfactory in the treatment of 
polarization of light bj' the different crystal systems. 
The common rock-forming minerals, as they occur in thin sections of 
rocks, are then described, beginning with those of the cubic system and 
