436 Prof. T. G. Bonney—On some Breceias and Crushed Rocks. 
“strain ”’), are produced (like fault breccias) by mechanical forces, but 
differ from them in some respects. The latter occur in the imme- 
diate neighbourhood of an obvious dislocation of the rock-masses 
and cut more or less athwart the general lines of bedding (where 
these can be observed). The former, however, not seldom retain a 
general parallelism with the bedding, though of course there are 
cases in the more uniform massive rocks where it is exceedingly 
difficult to be sure as to the precise cause of the formation. A crush 
breccia in short occurs along a plane at right angles to the direction 
of maximum pressure, while obviously a fault breccia which does 
not begin to form until disruption has taken place under strain is 
more nearly in the direction of pressure. The circumstances of the 
case also will often cause greater displacements of the fragments and 
give it a closer resemblance to an ordinary breccia, from which how- 
ever it is generally distinguished without much difficulty unless the 
fault happens to be small. 
The “crush breccia” appears to be the result of some accidental 
local weakness in a rock generally uniform—a portion yields to the 
pressure, and the equilibrium being once destroyed a lenticular 
mass is crushed in situ—the fragments—and this is important to 
remember (for Professor Hughes’ test cannot always be readily 
employed)—being sometimes turned about and relatively displaced. 
These breccias vary in size, but very commonly the fragments have 
diameters from an inch or two downwards. They may, however, be 
very small—say from a mustard seed to mere dust. In this latter 
case, of course they are most of all perplexing, but, commonly 
speaking, by patient work with the microscope the relations of the 
fragments and the true nature of the rock are to be discovered.' 
Some beds of schist apparently interstratified in more granitoid 
rocks, as I have already pointed out in more than one paper, are of 
this origin. In some cases, however, the crushing has taken place 
near a fault, and is in connexion with it; but still the rock resulting 
is of the type which I am describing. 'These crush breccias (in- 
cluding the minute forms) so far as my experience goes, are especially 
common in the oldest rocks, and in the more granitoid varieties of 
them. J have examined numerous specimens from the Scotch High- 
lands, collected by Drs. Callaway and Hicks, by Prof. Lapworth and — 
myself. They occur in the massive gneisses of the Alps (I suspect 
very frequently the lenticular schist bands apparently interbedded 
are thus produced, but I have not yet had time to work out micro- 
scopically a considerable collection made both during the present 
summer and in previous journeys). This breccia in the St. Davids 
“ Dimetian ” (undoubtedly a very ancient rock, whatever its true 
nature may be) is another case. I once found a breccia near the top 
of the North Hill, Malvern, which I have now no doubt is another 
case, and it would be easy to increase the list. It must be remem- 
bered that very commonly micaceous, chloritic, and other minerals 
have been subsequently developed, which enhance the resemblance 
to an ordinary schist. 
1 Mr. Clough once sent me some remarkable instances from granites in the Cheviots. 
