PROCEEDINGS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. xlv 
science, where the structure and comparison of the different organs 
of the animal or plant are considered apart from their functions or 
their growth. 
Our Type Rock Collection, then, is intended as an introduction 
to the study of Petrology, and amongst the lessons which it is 
intended to teach are—(i) What are the principal kinds of rock 
which go to build up the solid crust of our globe; (2) how are these 
different kinds related to each other, and how may they be classified 
in a logical sequence; and (3) how do they differ from each other in 
mineral constitution and in macroscopic and microscopic structure. 
These subjects of enquiry we have endeavoured to illustrate by means 
of carefully-selected specimens, full explanatory labels, and photo¬ 
graphs and coloured drawings. In studying the collection, the first 
fact to be grasped is that all rocks are divided broadly into three main 
groups, resultant on their mode of origin, although, strictly speaking, 
Petrology does not concern itself with the 7 nodus operandi of their 
origin. These groups are—1st, rocks of accumulation, or sedimen¬ 
tary rocks; 2nd, altered, or schistose rocks; and 3rd, rocks which 
have once been in a molten condition, or igneous rocks. The first 
of these groups is sub-divided under five headings, according to the 
nature of the accumulated material, whether it be of (1) sand, (2) 
clay, (3) volcanic debris, (4) material derived from animal or plant 
remains, or (5) from the precipitation of various chemicals by the 
evaporation of the water in which they were dissolved. The third 
great group, namely the igneous rocks, is sub-divided—somewhat 
arbitrarily perhaps—into four sections, according to the preponder¬ 
ance of one particular mineral substance, namely, silica, or silicic 
acid. These sections are called respectively the acid, intermediate, 
basic, and ultra-basic series. In the first we have, of course, the 
greatest proportion of silica, and in the last the smallest. 
The second great group, namely, that of the schistose rocks, occupies 
a somewhat anomalous position in the petrological series, as the rocks 
which it embraces have all originally been normal sedimentary or 
normal igneous rocks, while in many cases it is difficult to say whether 
they were originally the one or the other. For this reason I have 
placed the group in an intermediate position between the two in the 
collection, although in most text-books, and, therefore, in most 
museums, its place is at the end of the classification. 
Now, assuming that a visitor to the museum, of ordinary intelli¬ 
gence, but entirely ignorant of the principles of geology, should go 
to this collection for the purpose of learning something about the 
different kinds of rock, let us see what it is capable of teaching him. 
It may be taken for granted that to the casual observer, either in 
town or country, one rock is very much like another, particularly as 
what he sees—whether it be in the wall of a house or the side of 
a hill—is generally a more or less weathered surface. In other 
words, the phrase “ a stone ” covers a vast variety of structure, com¬ 
position, and complexity, which to the popular mind differentiates 
itself into little more than “Freestone” or “YVhinstone.” Such a 
visitor, however, going to Table Cases C and D in the Index Museum, 
