500 J. Allan Thomson—Inclusions in Volcanic Rocks. 
The inclusion of isolated minerals in tuffs and volcanic necks has 
puzzled many students of volcanic phenomena. In the first place 
their origin is obscure, as often they cannot be traced to any known 
rocks; and in the second place, even if they are known in the 
associated igneous rocks, their size is still a puzzle. Sir A. Geikie 
says:' ‘The crystals in question seem, however, to point to some 
chemical process still unknown, which, in the depths of a volcanic 
focus, under conditions of pressure and temperature which we may 
speculate about but can perhaps hardly ever imitate in our laboratories, 
leads to the elaboration of the diamond, garnet, sahlite, smaragdite, 
zircon, and other minerals.” The case of the Kakanui breccia shows 
at least that the majority of these minerals so often found in tuffs, 
including the refractory garnet, do form in homceogenous nodules from 
the volcanic magma. One may even speculate that the origin of 
the minerals of the Kimberley diamond pipes is due to the same 
process, and that the associated rocks may be in large part of the 
nature of homceogenous inclusions. 
Homeogenous Inclusion in Greenstone Sill, Cudden Point, Cornwall. 
In his discussion of inclusions Lacroix has wisely confined himself 
almost entirely to the younger volcanic rocks in order to eliminate all 
phenomena due to later alteration. Now that his conclusions are well 
established it is of interest to study those of the older volcanics as 
well. The West Cornish greenstones are very free from inclusions, if 
we except veining from the granite contacts, but in that of Cudden 
Point I have found large blocks of homceogenous inclusions. The sill 
itself has been profoundly affected by pressure and secondary changes, 
and near its lower boundary consists practically of a schist formed of 
fibrous hornblende and zoisite, but the original rock was probably an 
ophitic dolerite. The inclusions have suffered also from these changes, 
but not nearly to the same extent. They have, as it were, formed 
eyes (Augen) during the: production of the schist. They still contain 
the original augite, ophitically enclosing large plates of plagioclase 
felspar, which is the predominant element of the rock. In the 
absence of Carlsbad twins Levy’s exact method is not available, while 
the largest angles of extinction observed on symmetrically lying 
albite-lamelle amount to 12°. This would indicate a basic oligoclase, 
but it is unlikely that the felspar is so acid. One or two of the 
crystals show a slight zonary banding. 
_ The effects of crushing are seen in the bending of the lamelle of 
the felspars and local comminution with production of a granulitic 
structure. One augite crystal is neatly faulted. 
Secondary changes are the production of numerous granules of 
zoisite in the felspars, and in the growth of a colourless fibrous 
hornblende from the augite along its boundaries and in its cleavage. 
cracks. Small fibres exist also in the felspars. 
1 Loe. cit., vol. 1, p. 62. 
