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Geological Survey of Great Britain. 233 
had to be considered, viz., the degree of porosity, as influencing 
the weight of coal which can be stowed in a small compass, 
and the strength of the fragments, which ought to be great 
enough to prevent their crumbling under the influence of the 
shocks produced by the motion of the vessels, &. 
It was resolved, with a view to satisfy all conditions, to 
undertake not only chemical experiments, but really practical 
trials. The Lords of the Admiralty made over an annual 
sum of £600 to cover the expenses; and, under the direction 
of Playfair and Phillips, a boiler was erected for the purpose 
at the Polytechnic College, founded about three years ago at 
Putney, and the investigations were commenced, the results 
of which have been published in a First Report in the summer 
of 1848. 
Execution of the Operations—Maps and Sections. 
The persons entrusted with the Geological Surveys, the as- 
sistants, geologists, and directors, work separately, although, 
in general, at no great distance from each other. Each re- 
ceives two copies of the map of the district he is to examine, 
one for entering his observations in the field, the other 
for copying down the results obtained. The survey is ef- 
fected, as far as possible, by directly following the boundary 
lines in nature ; but where this is impossible from the inter- 
vention of cultivated ground, it is considered allowable to 
draw the most probable line by connecting the two nearest 
points of observation. We had an opportunity of seeing the 
exactness with which they go to work on these observations, 
when accompanying Mr Ramsay, who, in this autumn, under- 
took the survey of the valley of Llanberris in North Wales. 
The flanks of Snowdon, facing this valley, consist partly of ig- 
neous rocks, partly of altered sandstone and slates, which being 
mingled with the products of submarine eruptions, lava, ashes, 
&c., and altered by subsequent metamorphoses, have often lost 
nearly every trace of stratification, and every sign of a neptu- 
nian origin. Itis a labour not less difficult than (if considered 
by itself) thankless, to follow up the complicated lines of 
boundary between the two kinds of rock, over the bare and 
steep crags, which are frequently not to be attained without 
