770 REPORT—1896. 
‘work yet remains to be done in this connection, especially concerning horizontal 
distortion of masses of the earth’s crust, owing to more rapid horizontal advance 
of one portion than of another, during periods of movement. Not until we gather 
together a large amount of information derived from actual inspection of the rocks 
shall we be able to frame satisfactory theories of earth-movement, and in 
the meantime we are largely dependent upon the speculations of the physicist, 
often founded upon very imperfect data, on which is built an imposing super- 
structure of mathematical reasoning. We have been told that our continents and 
ocean-basins have been to a great extent permanent as regards position through 
long geological ages; we now reply by pointing to deep-sea sediments of nearly 
all geological periods, which have been uplifted from the ocean-abysses to form 
portions of our continents ; and as the result of study of the distribution of fossil 
organisms, we can point almost as confidently to the sites of old continents now 
sunk down into the ocean depths. It seems clear that our knowledge of the causes 
of earth-movements is still in its infancy, and that we must be content to wait 
awhile, until we have further information at our disposal. 
Recent work has proved the intimate connection betwixt earth-movement and 
the emission and intrusion of igneous rocks, and the study of igneous rocks has 
advanced beyond the petrographical stage ; the rocks are now made to contribute 
their share towards the history of ditlerent geological periods. The part which 
volcanic action has played in the actual formation of the earth’s crust is well 
exemplified in Sir Archibald Geikie’s Presidential Addresses to the Geological 
Society, wherein he treats of the former volcanic history of the British Isles.‘ The 
way in which extruded material contributes to the formation of sedimentary 
masses has, perhaps, not been fully grasped by many writers, who frequently seem 
to assume that deposition is a measure of denudation, and vice versd, whereas 
deposition is only a measure of denudation, and of the material which has been 
ejected in a fragmental condition from the earth’s interior, which in some places 
forms a very considerable percentage of the total amount of sediment. 
The intruded rocks also throw much light on past earth-history, and I cannot 
give a better illustration of the valuable information which they may furnish to 
the stratigraphical geologist, when rightly studied, than by referring to the 
excellent and suggestive work by my colleague, Mr. Alfred Harker, on the Bala 
Voleanie Rocks of Carnarvonshire.” 
Perhaps the most striking instance of the effect which detailed stratigraphical 
work has produced on geological thought is supplied by the study of the crystal- 
line schists. Our knowledge of the great bulk of the rocks which enter into the 
formation of a schistose complex is not very great, but the mode of production of 
many of them is now well known, and the crude speculations of some of the early 
geologists are now making way for theories founded on careful and minute obser- 
vations in the field as well as in the laboratory. Recent work amongst the erystal- 
line schists shows, furthermore, how careful we should be not to assume that 
because we have got at the truth, we have therefore ascertained the whole truth. 
We all remember how potent a factor dynamic metamorphism was supposed to be, 
owing to discoveries made in the greatly disturbed rocks of Scotland and Switzer- 
land; and the action of heat was almost ignored by some writers, except as a 
minor factor, in the production of metamorphic change. The latest studies amongst 
the foliated rocks tend to show that heat does play a most important part in the 
manufacture of schists. The detailed work of Mr. George Barrow, in North-east 
‘Forfarshire,’ has already thrown a flood of light upon the origin of certain schists, 
and their connection with igneous rocks, and geologists will look forward with 
eagerness to further studies of the puzzling Highland rocks by this keen observer. 
The subject of former climatic conditions is one in which the geologist has 
very largely depended upon followers of other branches of science for light, and 
yet it is one peculiarly within the domain of the stratigraphical geologist; and 
) Sir A. Geikie, Quart. Journ. Geol. Soc., vols. xlvii. and xlviii. 
2 Alf. Harker, Sedgwick Essay for 1888 (Camb. Univ. Press, 1889). 
3 G. Barrow, Quart. Journ. Geol. Soc., vol. xlix. 1893, p. 330. 
