452 SL GIDMMES IROOM SIUM EIN IS 
have ordinarily been regarded exclusive of each other, but it is 
believed that they may be mutually supporting. 
My microscopical study of both cleavable slates and schists 
has convinced me that in the interstices, and by the decomposi- 
tion of the larger particles, new minerals, and especially mica, 
abundantly develop with similar orientation and with their longer 
diameters or cleavage, or both, parallel to the flattened or rotated 
original particles. The innumerable parallel minute flakes of 
cleavable minerals in slate, especially mica and chlorite, which 
are almost universally present, are in no case detrital, so far as 
observed by me, but have developed in situ. Usually it is easy 
to discriminate the large, comparatively sparse, fragmental mica 
plates, if any are present, from those which are autogenic. As 
soon asa new mineral particle has developed it is subjected to 
flattening and rotation precisely as is an original mineral particle. 
This parallel arrangement of minerals developed in situ is prob- 
ably the most important single cause of cleavage. 
Not infrequently unmodified igneous rocks have the property 
of rift or cleavage more or less perfectly developed. In all 
‘cases observed by me this capacity is due to the arrangement of 
‘the mineral particles with their longer diameters in a common 
direction or to their similar crystallographic orientation, or both. 
In the case of cleavable minerals, the particles which are simi- 
larly oriented give the rocks a capacity to part parallel to the 
direction of readiest mineral cleavage, and this tendency is more 
‘marked if the greater dimensions of the mineral particles accord 
with their cleavage. In some cases, in which there is a rift in 
two directions, this is due either to cleavage in the mineral par- 
ticles in two directions or to cleavage of them in one direction and 
their parallel arrangement with longer axes in the other direction. 
Hornblende is one of the minerals which sometimes produces a 
cleavage by having very numerous crystals with their longer 
diameters in a common direction. Feldspar is one of the min- 
erals which produces cleavage on the same principle as horn- 
blende, but which also in some cases gives a rock-cleavage as a 
result of the cleavage of the mineral particles. 
