76 
DINTS To TRAVELLERS, 
in general have been long since removed by other denuding agencies. 
The most important evidence of former ice action consists in the 
occurrence, embedded in fine sediment, of large boulders, occasionally 
preserving marks of polish and striation, and usually, though not always, 
angular. Accumulations of this kind afford evidence of transport by 
two different agencies, water, which has brought the silt, and ice, which 
has carried the boulders. If the water had been in rapid movement, 
and thus capable of moving the boulders, it would have carried away 
fine silt or sand, instead of depositing it. Evidence of ice action has 
thus been traced equally in the boulder clay of North-Western Europe, 
and in the Palaeozoic boulder beds of India, South Africa, and Australia, 
and probably of South America. 
It is well to search in all mountain ranges for traces of glacial 
action. In many mountain chains, even in comparatively low latitudes, 
proofs have been found of the existence of glaciers, at a much lower level 
than at present, dating from a comparatively recent geological period, 
whilst in other mountain regions none have been recognised. The ques¬ 
tion also whether glacial action has been contemporaneous in the two 
hemispheres is of the greatest importance, and the evidence hitherto 
adduced is of a very conflicting character. 
Deserts .—The great sandy or salt plains, with a more or less barren 
surface, that occupy a large area in the interior of several continents, have 
only of late years received due attention from geologists. A great thick¬ 
ness of deposits must occur in many of these vast, nearly level, tracts, for 
the underlying rocks are often completely concealed over immense areas. 
The investigation of the deposits is frequently a matter of great difficulty 
for want of sections; but, where practicable, a careful examination should 
be made, and exact descriptions of the formations exposed recorded. 
Some, at all events, of these beds appear to be entirely deposited from the 
air, and consist of the decomposed surfaces of rocks and the sand and 
silt from stream deposits, carried up by wind and then redeposited on the 
surface of the country. Such deposits are very fine, formed of well- 
rounded grains, and, as a rule, destitute of stratification. The geologist 
who has especially described these formations, Baron E. von Bichthofen, 
in his work on China, attributes to the loess of the Rhine and Danube 
valleys a similar origin. It is usual to find beds due to water-action, 
rain-wash and steam-deposits, interstratified with the subaerial accumu- 
