[ 273 ] 
VI. On a Collection of Rock Specimens from the Island of Socotra. 
By T. G. Bonney, M.A., F.R.S., F.G.S., Professor of Geology in University College, 
London, and Fellow of St. John's College, Cambridge. 
Received June 12,—Read June 15, 1882. 
[Plates 6-7.] 
The specimens of rocks—about 500 in number—collected by Professor Bayley 
Balfour during bis late exploration of the Island of Socotra, were forwarded to me 
for examination. Several of these specimens, as was to be expected under the circum¬ 
stances, were in a condition unfavourable for precise determination, being often frag¬ 
ments from weathered surfaces and sometimes much decomposed. Each, however, has 
if*. 
been described as far as the circumstances would admit, and microscopic slides have 
been prepared for me by Mr. Cuttell from about 80 of the more interesting. 
As it happens, certain of these offer difficulties which in the present state of our 
knowledge are almost insuperable. While the use of the microscope has dispelled 
much confusion in our petrological ideas and supplied us in many respects with a firm 
basis of knowledge, it has not in every case—owing to the novelty of this mode of 
research and the inherent difficulties—enabled the student to feel perfect confidence in 
some of his conclusions, especially when he is restricted to this method of examination. 
Perhaps the greatest of our petrological difficulties is the distinguishing in every case 
between certain highly metamorphosed rocks and those of similar chemical composi¬ 
tion which are truly igneous. It has, indeed, been maintained by some eminent 
geologists that certain sedimentary materials may be so altered by the combined 
action of water, heat, and pressure as to be converted in situ into a rock indistinguish¬ 
able from one of those commonly held to be of igneous origin. Accordingly we read 
not seldom of “ metam orphic granite” and of “gneiss passing into granite,” for it is of 
these that the above opinion is commonly held. Other geologists, indeed, go yet 
further and make a similar assertion, not only of the more coarsely crystalline rocks 
such as syenite, diorite, and gabbro, but even of the more compact varieties of felstone 
and greenstone, which in like manner are said to afford indubitable examples of tran¬ 
sition into beds of true sedimentary origin. As regards this view, we may admit that 
if a mass of clastic materials be once reduced to a molten magma its past history is 
obliterated; and, further, that there is no reason, so far as we know, why this melting 
down should not occur. In this sense, any igneous rock whatever may possibly 
deserve the name of metamorphic. But, while admitting the d priori possibility of 
MDCCCLXXXIII. 2 N 
