166 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
phenomena seen in the rocks. Of all the crystalline rocks found in 
the Highlands, the hornblende schists seemed to most fully confirm 
the theory that these rocks had been precipitated from the waters of 
some thermal ocean during the earlier ages of the world’s history. 
But the facts we have just brought to light tend to show that we have 
nowhere evidence for assuming that any of these rocks are part of the 
primitive crust of the earth or that they have been formed under 
conditions different from those with which we are acquainted. 
The following is a summary of the various stages through which 
the hornblende schist must have passed before assuming its present 
appearance:— 
(1) The hornblende schist must have been originally intruded 
into the clastic schists as a dyke or sill of basic igneous rock, at a 
period prior to the metamorphism of either rock. 
(2) During this intrusion it caught up and enveloped large and 
small masses of the clastic rocks, the intrusive sill being thus con¬ 
verted into a heterogeneous mass. 
(3) During the great period of regional metamorphism which 
subsequently affected the rocks, while the clastic rocks were meta¬ 
morphosed into schists, the basic igneous rock was metamorphosed 
into a true hornblende schist, the rock undergoing the following pro¬ 
found chemical and structural changes :—f a) Chemically, the felspars 
passed into sausurite, while the augite became changed into horn¬ 
blende. (h) Structurally, the original crystals of felspar became 
broken up into granular masses, resembling those of a clastic rock, 
while the crystals of augite passed into long needle-shaped prisms 
of hornblende. (c) The masses of grit and other clastic rocks 
included in the basic igneous rock were drawn out into bands of 
varying thickness, depending upon the original size of the included 
masses, thus giving the hornblende schist a perfectly banded 
structure. 
XVIL —Recent Advances in the Study of the Rocks of Highland 
Perthshire. 
By Peter Macnair. 
(Read 8th April, 1897.) 
The remarkable confirmation which the famous researches of 
James Nicol on the structure of the north-west Highlands received 
at the hands of Professor Lapworth and the officers of the Geological 
Survey has now become a matter of history. 
Nicol’s paper was originally read on 5th December, i860, and 
