474 Proceedings of Philosophical Societies. (Junx, 
Lead has become oxidated, and some small pieces are intermixed 
in a mass of it which appear to be galena. The sulphur in this 
case the author considers to be obtained by the depuration of the 
lead from long exposure to heat, the uncommon softness of the 
metallic lead being a circumstance to support the conjecture. 
There are also specimens of compact minium derived from 
common shot. 
March 7, 2\.—At these meetings a paper by Dr. Macculloch, 
entitled, Corrections and Additions to the Sketch of the Mineralogy 
of Sky published in Vol. IL], of the Transactions of the Geological 
Society, was read. 
In a visit to the island in the course of last summer Dr. Maccul- 
loch was enabled to continue his examination of its structure, and 
to detect some errors which had occurred in the former paper, occa- 
sioned partly by the inaccuracy of Mackenzie’s chart, which was 
his guide in his first journey. 
In the paper already before the Society the promontory of Sleat 
was stated to consist of micaceous schist as the lowest rock of the 
island. ‘To this sueceeded greywacke and schist, then quartz rock, 
and afterwards red sand-stone. In the present examination Dr, 
Macculloch has found that the beds of gneiss alternate with and are 
in greater proportion than the mica-slate. This gneiss passes into a 
rock composed of felspar and quartz, with chlorite schist interlami- 
nated together, the latter being substituted for the mica of the re- 
gular varieties. Decided alternations of the various rocks connect 
the red sand-stone with these beds, and offer an extraordinary in- 
stance of the connexion of the red sand-stone with a primary rock— 
the gneiss. 
Among the lyas lime-stone beds with shales and sand-stones inter- 
vening which lie over the red sand-stone, changes are met with that 
have converted the lime-stene into marble, and the sand-stone into 
quartz. On the western shore of the northern district the lyas is 
converted into chert, and the shale into siliceous schist. ‘These 
changes can be traced through various intermediate states. 
The rocks of Trotternish are those of the lyas formation inter- 
sected and covered by trap. In many places the trap appears inter- 
stratified with the beds below it. Often the alternations of the trap 
are as regular as those of the stratified rocks with which it is con- 
nected; but it invariably happens that after some distance the trap 
which has continued between two of the beds passes through the 
one or below, either uniting with the superincuimbent mass of trap, 
or passing for a further distance with similar regularity between two 
other strata. 
In some instances Dr. Macculloch has observed this to happen 
after the trap had continued in a regular course between two beds for 
more than a mile; and he concludes that all the supposed cases of 
alternations of the trap rocks with stratified ones are of a similar 
nature. 
The trap of Sky is generally amorphous; but at the northern end 
3 
