68 THE VICTORIAN NATURALIST. 



there are many different kinds of felspar, each with a name of its 

 own, and all differing in chemical composition and in crystalline 

 form. Still they are closely enough allied to render their grouping 

 under the one term, felspar, advantageous. They are all known 

 to chemists as highly complex silicates. The species of felspar 

 which occurs in basalt contains silicates of alumina, lime, and 

 soda, the proportions of which vary within certain fairly well- 

 defined limits. Usually in our lava-flows the crystals are too 

 small to be seen by the naked eye, but at some localities, as at 

 Malmsbury, they are as coarse as chopped grass. Felspars are 

 hard, and scarcely acted on by the usual acids of the laboratories, 

 but the long continued action of the comparatively weak carbonic 

 acid carried down by rain water will in time decompose them. 

 All the soluble parts will eventually be washed away, and the 

 insoluble residue, combined with water, forms clay. During the 

 progress of this decomposition other minerals will be formed, 

 which, under favourable conditions, may be deposited in cavities 

 in the rock. A common mineral yielded in this way by decom- 

 posing basalt is carbonate of lime, which may be temporarily 

 stored away under many forms. Thus at Collingwood one of the 

 forms of carbonate of lime, known as arragonite, is deposited in 

 sheaves of brilliant needles, which glint and sparkle in the sun- 

 light. Another form, known as calcite, occurs in truncheon-like 

 masses with roughened surfaces, and stained brown by a small 

 amount of iron in it. The amount of iron may be increased, and 

 the resulting mineral may entirely fill a cavity in the basalt, as in 

 the case of the banded, almond-shaped masses so common at 

 Riddell's Creek. 



The handsomest of all the minerals of the quarry, however, are 

 the zeolites. Like felspars, they are complex silicates, but unlike 

 felspars they contain a considerable amount of water. If heated 

 before the blow-pipe they easily fuse, and the escaping water boils 

 and bubbles, and hence the name, which means the boiling 

 mineral. Owing to great variability in their composition, their 

 classification is very difficult chemically, so that they are grouped 

 on their crystalline form. It will probably be enough to mention 

 the commoner ones, such as phacolite, mesolite, and phillipsite. 

 The cavities in which the finest specimens occur are often pecu- 

 liar, and are found deep down near the quarry floor as well as 

 nearer the surface. The spaces are such as would be left in 

 an egg-cup if we put into it an egg somewhat too large to fit 

 closely. But in the quarry these spaces are turned the other way 

 up, so that we may come across a dome-shaped mass about as 

 large as the top of one's head, and on this mass the zeolites and 

 carbonates occur. There is usually a mossy, velvet-like coat of 

 arragonite crystals, with here and there one of the truncheon- 

 shaped calcite crystals of a dark brown colour. Scattered about 



