378 Correspondence. 
unless it can be shown that they are advancing with the dip of 
soft-bedded strata which flank the harder rocks that form the 
mountains. 
' I believe that instances of this kind have occurred in New 
Zealand, especially on the west side of the mountains. 
The great valleys, however, which exist in the Crystalline rocks, 
and which are ‘rock-basins,’ have become so by unequal axial motion, « 
which, in the case of Otago, has generally been a subsidence of the 
mountain-centres, more rapid on the west than the east, and accom- 
panied by arise of the flanking formation. However, I can have 
no doubt that in the schistose formations, and other stratified rocks 
having alternations of hard and soft beds, the valleys have been 
excavated in the strike of the soft strata by simple erosion. 
In the case of the massive Crystalline rocks, I have reason to 
suppose that the valleys, such as are occupied by the true coast- 
sounds, have originally followed lines of injected basic rocks, that 
invaded the crystalline mass subsequent to its original metamorphism, 
and which basic rocks, decomposing more freely than the gneissic 
group on the subsequent elevation of the land, gave way before the 
descending ice. It seems to me that the essential condition to the 
existence of glaciers is a lofty plateau, or slightly inclined mountain- 
top, on which the snow rests in sufficient quantity to descend into 
the lower levels as its only means of escape. The greatest erosion 
takes place where it descends over the edge of this plateau, or 
‘ breeding-ground ;’ and if there is a soft ‘run’ in the rocks, it must 
cut a notch there which will extend into the higher country as a 
deep valley. I believe I can prove that it has been thus that all 
our principal valleys have been formed. Ice is only, after all, one 
of the ordinary destructive forces, the effect of which has, perhaps, 
been rather underrated ; but there has been no new erosive agency 
added to that formerly known to geologists. The ice-fall acts just 
like a great cascade in a river, at which point the greatest erosion 
goes on—that which is effected by the gently running part of the 
stream being small. The only supposed new point in these dis- 
coveries is the excavation, or ploughing action, by the ice of the 
glacier after it has descended to the valley-level, and with that I 
certainly do not agree. 
Trusting that I can shortly support my views with full details, 
Tremain, my dear Sir Roderick, yours sincerely, 
JAMES HECTOR. 
Dounevin: April 18, 1865. 
GEOLOGY OF MATLOCK AND THE NEIGHBOURHOOD. 
To the Editar of the GEOLOGICAL MAGAZINE. 
Sir,—Allow me, through the medium of your Magazine, to direct 
the attention of Geological tourists during the ensuing autumn to 
some interesting spots in a part of Derbyshire not yet sufficiently 
known. The Mountain-limestone around Matlock presents very 
instructive sections, which have lately been extended by quarrying. 
