22 Transactions of the Society. 



III. — On the Preparation of Sections of Pumice-stone and 

 other Vesicular Bochs. 



By H. J. Johnston-Lavis, M.D., F.G.S. 



(Read llih November, 1885.) 



The art of making sections of rocks of a compact structure, or even 

 slightly vesicular lavas, presents no other difficulties than those 

 which have now been overcome ; but when thin slices of pumice- 

 stone or very buUate scoria are required, many difficulties arise. 



In the first place, when the section reaches a moderate degree 

 of thinness, it becomes an open network of substance that is very 

 fragile, and the strain put upon the delicate trabecules by the 

 friction of grinding breaks them down long before the requisite 

 thinness is reached. In the case of pumice, unless the section is 

 very thin, httle can be learnt, on account of the darkness and 

 clouding produced by the still unopened minute air-cavities. 

 Another important difficulty is due to the different resistance of 

 large crystals and the comparatively soft vitreous or microcrystal- 

 line base in which they are imbedded. In consequence of this, the 

 very feeble support of the setting (so to speak) is insufficient to 

 resist the grinding action, so that the crystals are torn out and 

 plough a line quite across the preparation. The third difficulty is 

 that any pulverulent substance, such as emery, must be avoided, 

 since new cavities are continually being opened, which get choked 

 with the detritus and spoil the preparation for examination. 



In most books we are recommended to boil the pumice in 

 Canada balsam, but a moment's thought will prevent our spending 

 any time over such an experimental failure. We know that the 

 vesicular cavities in pumice or scoria are closed except at the 

 surface, where they are fractured, and therefore a balsam bath can 

 only enter these superficial cavities, and immediately one com- 

 mences to grind, fresh ones will be opened, whose walls are left 

 unsupported by balsam. The method I have adopted is the out- 

 come of a long series of experiments, by which I have produced 

 many dozens of excellent slides, even from the most fragile 

 pumices. 



The pumice may be cut into a moderately thin slice by a 

 saw (about • 5 cm. is convenient to work with), or if an abund- 

 ance of material is at hand a level surface may be obtained with 

 a knife ; if scoriaceous pumice or scoria, a well- watered grindstone 

 may be used. The sawdust or other dust must be brushed, 

 blown, or washed out of the inequalities in the moderately level 

 surface, and the slice placed on a hot plate to dry and warm. 

 When thoroughly dry, and while still on the hot plate, a stick 



