of Edinburgh, Session 1872-73. 139 
it will be seen that the boulders are divisible into two classes, — 
rounded and angular. 
The boulders referred to are, of course, all t( erratics,” in the 
geological sense of the term, i. e., they have been transported for 
considerable distances from a parent rock. 
The rounded boulders are generally composed of rocks extremely 
tough and hard, and on this account were capable of undergoing 
great friction and rough usage without being broken up. Hence the 
round-shaped boulders most frequently consist of blue whin-stones, 
fine-grained granites, schists, limestones, and felspathic rocks. 
The angular-shaped boulders, whilst embracing these rocks 
embrace also sandstones and conglomerates. Their angular shape 
of itself proves they could have undergone little or no rough usage 
by being rolled or pushed, for their angles and corners are in 
many cases sharp. Moreover, the rocks composing them are so 
loose and friable in texture, that they would have crumbled or 
been crushed to pieces liad they undergone rolling or pushing. 
The difference in shape between the two classes of boulders now 
referred to can be seen by looking at figures I., II., and V., as 
contrasted with figures IV., VI., and XIII. in Plate. 
It may here, however, be proper to explain that some of the 
boulders composed of the friable rocks just mentioned, whilst 
angular and rough on one side, are sometimes rounded and smooth 
on the other, — a fact apparently indicating that after being carried 
to their present position without injury or mutilation, they had 
been subjected to friction or attrition on the particular side which 
is now smooth. 
Some angular boulders are not cubical in shape, but are longer 
than they are broad ; and in that case, the smoothed end, when 
there is one, is almost always narrower or more pointed than the 
rough end. It was a remark of Hugh Miller’s, that on examining 
pebbles lying in the channel of a river, the great majority have 
their narrow ends pointing up stream. When once in that position, 
they retain the position longer than when lying broadside against 
the stream. This remark should not be lost sight of, in drawing 
inferences from the shapes and positions of boulders. 
Examples of these angular boulders, with one end smoothed and 
pointed, will be noticed in Sketches VIII. and X. 
