xlvi PROCEEDINGS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
will see about 130 pieces of rock, roughly trimmed with the hammer 
to a uniform size, so as to expose a fresh and unweathered surface,, 
and all obviously different from each other. If he goes at random 
even to types which are comparatively familiar to him, the probability 
is that he will learn something about them that he did not know 
before. To take a few familiar examples,—if he goes to the specimen 
of sandstone, he will see, first, that it is placed in the group of rocks 
which are built up from the ruins of older rocks, that the particles 
of which it is composed were once borne along by running water 
and then deposited as sand on the shore of an ancient sea, and, 
lastly, that the particles of sand have been cemented together by a 
flinty deposit, stained a red or yellow colour by the presence of iron. 
Next, if he goes to the group of rocks which are so familiar in the 
highland parts of our county, both as native rock and as building 
stone, namely, the various kinds of schist, he will find that these, 
too, have once been laid down by running water, as the sediment 
which it could no longer carry, but that, long ages after it had 
hardened into a sandstone, its particles of quartz and mica got 
squeezed and contorted in such a way that they were forced to re¬ 
arrange themselves in the beautiful wavy lines which are largely 
accountable for the grandeur of our Highland scenery. Going 
round the corner to the north side of the case he comes to the 
granites, and here he will note that he has reached the group of 
rocks which have come under the influence of the internal heat of 
the globe, so that they were once reduced to a molten condition, 
and from that condition they have assumed their present form and 
structure by a process of crystallisation. In the hand specimen before 
him the crystals of the three predominating minerals—quartz, mica, 
and felspar—can be distinctly traced, but the crystalline forms are 
still more evident in the photograph of a microscopic section which 
is placed beside the specimen. From this photograph it will be seen 
that every particle of material has been crystallised. Going farther 
down the series of igneous, or fire-formed, rocks he will come to 
basalt, the typical “Whinstone” of the builder. Here the individual 
crystals are nearly microscopic, and therefore hardly observable in 
the hand specimen, but a coloured drawing from a microscopic 
section reveals their presence. This drawing also shows that part of 
the original glassy structure of the molten rock still remains. 
These are but four of the more familiar types, which I have 
chosen for the purpose of showing how properly-selected specimens, 
accompanied by fully-descriptive class and individual labels, and by 
structural drawings, may be made to teach the elements of any branch 
of natural science. If, on the other band, the specimens are not 
selected with special reference to the lesson which they are intended 
to teach, and if they are not arranged in logical sequence, and 
labelled according to a uniform and complete system, the result in 
the mind of the student is only confusion worse confounded. I use 
the word “ student ” here, not in the narrow sense of the votary of 
any particular branch of science, but in the broader sense that every” 
visitor to a museum or a library is, consciously or unconsciously, a. 
