clxxvi PROCEEDINGS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
in 1827 that William Nichol of Edinburgh perfected the method of 
preparing transparent sections of rock for examination under the 
Microscope. Since that time the microscopical examination of rocks 
has gone on steadily and laboriously, especially in Germany, so that 
now we know the crystallogical structure as well as the chemical 
composition of an immense variety of rocks. The chief value of 
these researches, however, has been the light which they have thrown 
on the past history of many rocks, and the cycle of changes which 
they have undergone in past ages. 
Another department of Geology which has been worked out 
almost entirely within the present era is that which relates to 
Glaciation. It was in 1840 that Louis Agassiz, the great Swiss 
Naturalist, first detected ice scratching and perched boulders in his 
native valleys, far beyond the range of the present glaciers. In 1850 
he visited this country, and, along with Professor Ramsay, detected 
the same phenomena here. It was not until 25 years later, however, 
that the subject was worked out in all its fulness by Dr. James 
Geikie, whose “Great Ice Age” was published in 1874. In the 
following year Dr. Croll published his work on “ Climate and Time,” 
in which he endeavoured to explain the astronomical causes which 
led to the cooling of the earth’s surface in glacial times. Dr. Croll’s 
arguments were extremely ingenious, but recent investigations, and 
particularly those of Mr. E. P. Culverwell, have led Physicists 
slightly to modify their views on this subject. 
Since the time when Lyell enunciated his Uniformitarian doctrine, 
a controversy has from time to time gone on between Geologists and 
Physicists regarding the age of the Earth. It was evident that if all 
the thousands of feet of sedimentary rocks had been built up by a 
slow process of denudation and deposition, a very long space of time 
was required for this to be accomplished. At first, perhaps, the 
Geologists were rather extravagant in their demands, and Lord 
Kelvin, with other Physicists, tried to limit them within a reasonable 
number of millions of years. Quite recently, however, Mr. Poulton 
has shown that the cooling of the Earth’s crust must have been 
a slower process than was at one time supposed, and has thus restored 
to the Geologist some of his stolen ages. Regarding the interior of 
the Globe itself we still know very little, although it is evident that at 
a comparatively short distance from the surface the heat must be so 
great that the rocks would be reduced to a liquid condition were it 
not for the pressure of the superincumbent mass. This fact has 
thrown considerable light both on Volcanic Phenomena and on the 
history of the Metamorphic rocks. With regard to the former, the 
most important work which has been published within recent years is 
“The Ancient Volcanoes of Great Britain,” by Sir Archibald Geikie, 
in which he demonstrates for Volcanic rocks what Lyell demonstrated 
for the sedimentary series, namely uniformity of the processes at work 
throughout all Geological time. 
With regard to Geography, which is now properly looked upon 
as a sister science to Geology, it is impossible even to glance at the 
additions which have been made to our knowledge as regards new 
territory explored during the Victorian Era. The word “ unexplored ” 
