PROFESSOR HEDDLE ON THE MINERALOGY OF SCOTLAND. 545 
Serpentines, as a rule, are denser, more solid,—give fewer and feebler 
evidences of being the result of change effected upon pre-existent rocks, when 
examined in their depths than on the exterior. Without entering upon the 
many and complex reactions which take place as secondary results of the inter- 
changes already noticed, it may be sufficient to show that the products of 
surface change, as they percolate through the deeper portions of a rock, may 
effect a somewhat dissimilar change therein, and may also, in their passage 
through these rocks, plug up their pores. 
Silicate of lime, formed by the action of meteoric water in the superficial 
portions of a rock, and meeting with magnesian salts within, would by inter- 
change supply serpentinous matter in the solid form to the more porous por- 
tions of the deeper-seated beds, to render the whole mass more uniform in 
structure, while it might also thereby be diversified in colour. 
When we remember that the decomposition of both augite and labradorite 
is effected through the operation of what may be called the ordinary agents of 
exposure—carbonic acid, oxygen, and water—but that, in virtue of their differ- 
ence in composition, the nature and rate of the decomposition varies, it is easy 
to explain how it is that the crystals of labradorite are protuberant from the 
general mass of the augite at Lendalfoot, while at Pinbain the augitic crystals 
stand in high relief above the felspar in that wave-washed situation. 
Augite, containing iron in the state of protoxide, most prone to higher 
oxidation, rapidly and readily gives way when subjected to aerial exposure, 
Where oxygen, carbonic acid, and water are alike free to operate upon it. 
Labradorite, containing little or no protoxide, is subject to the operation of the 
two last only, and so, in the air, is the more enduring. When plunged beneath 
water, however, the oxygen is in great part shut off from the augite, which is 
thus protected ; while here, the labradorite is subjected to the attack of water 
upon its alkaline silicates, suffering thereby rapid degradation. 
Varying circumstances, therefore, must ever vary the mode of 
degradation of a rock. 
From this consideration of the rationale of the process, elaborated at the 
desk, I turn to an illustration of its working, as shown in nature. 
That portion of the Long Island which is called Harris is almost entirely 
composed of a hornblendic gneiss, which has been assigned to Laurentian age. 
It is certainly the oldest of all known rocks in Scotland. 
In the northern portions of the district the rock dip is that normal to it on 
the mainland of Scotland,—namely, to the 8.S.W. 
This is well seen in the striking hills of Totam and Cleesham, which present 
great precipices to the north, and accessible slopes southward. 
In the south of the island, towards the Sound of Harris, the dip seems to. 
VOL, XXVIII. PART II. ae: 
