Reriews—Prof. A. C. Ramsay’s Address. 475 
_ the structures, even though the water was not the essential and active cause of the 
differences. 
Confining our attention to the more important crystalline constituents which are 
common to the different types, we may say that the chief structural characters of the 
erystals are as follows :— 
a. Skeleton crystals. d. Simple crystals. 
6. Fan-shaped groups. e. Fluid-cavities. 
ec. Glass-cavities. 
These different structural characters are found combined in different ways in the 
different natural and artificial products ; and for simplicity I will refer to them by 
means of the affixed letters. 
The type of the artificial products of fusion may generally be expressed by a + b 
or 5 + ¢, that is to say, it is characterised by skeleton crystals and fan-shaped groups, 
or by fan-shaped groups and glass-cavities. In like manner the volcanic type may _ 
be expressed occasionally by 6 + ¢, but generally by e+; and the granitic by d +e. 
These relations will be more apparent if given m the form of a table, as follows :— 
Slag type : : { ee ? Me 
Volcanic type { vine 4d 
Granitic type. : d+e. 
Hence it will be seen that there is a gradual passage from one type to the other by 
the disappearance of one character and the appearante of another, certain characters 
the meanwhile remaining common, so that there is no sudden break, but an over- 
lapping of structural characteristics. It is, I think, satisfactory to find that, when 
erupted rocks are examined from such a new and independent point of view, the 
general conclusions to which I had been led are so completely in accord with those 
arrived at by other methods of study. 
Conclusion —And now I feel that it is time to conclude. I have necessarily been 
compelled to give only a general account of the subject, and perhaps, for want of 
adequate description, many facts may appear more complex than they really are. 
Some are, indeed, of anything but simple character, and their full explanation is, 
perhaps, beyond our present power. The greater part are, however, much more 
simple and easy to observe than to describe; and, even if I have failed to make 
everything as plain as I could wish, I hope I have succeeded in making the principal 
point sufficiently clear to show that the structure of slags and of analogous artificial 
products throws much light on the structure and origin of the various groups of 
erupted rocks. I feel that very much still remains to be learned, and, as I think, 
could be learned, by the further extension of this method of inquiry. What strikes 
me most is the great necessity for the more complete application of experimental 
methods of research; but to carry out the experiments necessary to clear up the 
essential difficulties of the subject would, I fear, be a most difficult undertaking. In 
the meantime all that we can do is to compare the structure of known artificial 
products with that of natural rocks, and to draw the best conclusions we can from the 
facts, as viewed in the light of our present knowledge of chemistry and physics. My 
own impression is that there is still much to be learned respecting the exact conditions 
under which some of our commonest rocks were formed. 
REVIEWS. 
Seed is 
I.— British AssocraTION FOR THE ADVANCEMENT OF SCIENCE, 
Swansna, August 26, 1880. 
T has seldom, if ever, happened before, that the meeting of the 
British Association has been inaugurated by a purely geological 
_ address from its President. 
Whatever may have been that President’s special line of study, it 
has generally been the practice to furnish some retrospect of the 
work carried on in all the various branches of scientific research— 
besides geology. This year, however, the Presipent, Pror. A. C. 
