274 
PROFESSOR T. G. BONNEY ON A COLLECTION OF 
such a change, I must confess to being sceptical as to whether any case of it has yet 
been fully proved to exist. I have studied not a few of the alleged instances patiently 
and, as I trust, without prejudice, always with the fob owing results—either that there 
was no evidence which was conclusive on either side, or that to a practised eye there 
was very clear evidence against the asserted transition— i.e., that (to refer to the 
instance named above) either the so-called granite was clearly only a granitoid gneiss 
or else that it was distinctly intrusive in the schistose rock. In these investigations 
the microscope is a very great help, but I freely admit that there are many cases 
where we cannot rely upon it alone, and must also study the rock in the field. Our 
knowledge at present does not enable us to pronounce upon the classificatory value of 
certain structures which we observe in the microscopic study of some specimens. This 
difficulty, however, is one which time and experience will probably remove. To admit 
the existence of cases where it is at present safer to suspend the judgment in no way 
concedes that it is impossible ultimately to arrive at a conclusion. So then, while not 
professing in every case from examination of hand specimens alone to decide whether 
a rock is a granitoid gneiss or a true granite, I believe in the distinctness of the two 
rocks. As it happens, some of the specimens from Socotra belong to this dubious 
class; and I cannot say positively, even after microscopic examination, whether in 
certain cases we have a granitoid gneiss or a true granite, and in others a hornblendic 
rock of sedimentary origin or a true diorite. 
In drawing up my report on Professor Balfour’s collection I have thought it best 
to give, first, a general description of the petrology of those parts of the island over 
which he travelled, and then to describe the more important varieties of each group of 
rocks with which his journey has furnished us/"' 
The island of Socotra is about 72 miles long from east to west and about 22 miles 
in breadth. The general physical features are thus described by Professor Balfour 
in his account of the island printed in the volume of reports of the British Association 
for the year 1881 :— 
“ The surface features of Socotra at the present time are those of an island moun¬ 
tainous in the extreme. The shore line on its southern aspect is, as the map shows, a 
tolerably continuous one, unbroken by deep inlets or bays. On the northern side 
occur a few shallow bays at the mouths of the streams, which afford the only 
anchorage to be obtained around the island, but no one of them is safe at all seasons 
of the year. On all sides the hills rise with considerable abruptness over a wide area, 
forming bold perpendicular cliffs of several hundred feet in height, whose base is 
washed by the waters of the Indian Ocean, but at other places leaving plains varying 
in breadth up to as much as five miles between their base and the shore. On the 
south side of the island is the largest of these shore plains (Nogad), which, extending 
* In writing the first draft of this paper I followed the other plan, and described the specimens as 
they were collected by him during each section (generally representing a day) of his journey; but as I 
found this involve pi’olixity and needless repetition, I have recast the paper into its present form. 
