1888 .] 
Dr Stecher on Contact- Phenomena. 
165 
the “white trap” is but the result of the decomposition of diabase 
rock. The original identity of these two rock-types can also be very 
clearly demonstrated by examining the rocks under the micro- 
scope. The “ white traps ” all exhibit a microporphyritic structure 
and an ophitic ground-mass : the scattered crystals, often occurring 
in abundance, may be recognised by their outlines as having once 
represented olivine. These porphyritic crystals seldom show rounded 
forms; in such a case we are unable to say whether we are dealing 
with olivine or with augite (Newhalls). To sum up the description 
of these “ white traps,” it is chiefly to be noticed that nearly all of 
them occur in sheets of small thickness, and that, for the greater 
part, they are rich in disseminated olivine individuals. 
II. Modification in Form. 
(a) Modification of the Structure of the Rocks. — In the interior 
of the more extensive massive sheets, — as, for instance, of the 
Salisbury Crags, Hound Point, and Stewartfield, — the rock always 
exhibits a granitic or doleritic structure, while towards the Salband 
it assumes that of a microdiabase-porphyry, of which the ground-mass 
is ophitic. Passing through gradually finer-grained varieties, it is 
finally found as a true glass, in such places where, under the influ- 
ence of the associated rocks, the liquid magma was able to cool 
down with extreme rapidity. In spite of the great age of the 
diabases we are describing — Professor A. Geikie has pointed out that 
they are of Carboniferous age — a thin vitreous layer still exists at 
the outmost Salband of some rocks ; it is even to be detected at the 
Salband of two white traps, viz., from the shore west of St Monan’s 
Church and Kilmundy Quarry, where it forms a “ Gangbreccia.^' 
It is interesting to note that the glass is insoluble in hydrochloric 
acid. Finally, we may refer to the white traps, which, forming 
sheets of only small thickness, never possess the microdiabase- 
porphyritic structure. 
ifi) Modification of the Form and of the Structure of the Component 
Minerals. — The titaniferous iron-ore occurs in the rock-specimens 
from the interior of the larger massive sheets, as irregular patches, 
and, gradually diminishing in size as the distance from the centre 
increases, finally assumes the form of minute globulites. This tran- 
