220 



HARDWICKE'S SCIENCE-GOSSIP. 



is also a very general constituent of these granites, as is 

 also microcline. There are both white and black 

 micas the former being described as margarodite, by 

 the Rev. Dr. Haughton, who, judging from the mean 

 derived from several analyses, believes that the 

 amount of water of crystallization present differen- 

 tiates it from the variety known as muscovite. The 

 isolated masses before referred to, give good illustra- 

 tions of the hexagonal or lozenge-shaped plates, and 

 may be studied with interest, as the form in which 

 mica usually appears in granite does not afford the 

 information to a beginner. A peculiarity in the 

 felspathic and eurite portions of this granite, consists 

 in there being a number of crystals of a reddish-yellow 

 colour scattered through the mass. These are very 

 minute, none of them exceeding the size of a small 

 pin's head, but they are translucent, with a high 

 vitreous lustre. Closer inspection reveals certain 



^7—^ 



Fig. 126. 



Fig. 127. 



faces more or less imperfectly developed, owing to 

 the way they are imbedded in the matrix. The appli- 

 cation, however, of an inch power to the microscope 

 gives a fuller insight into the forms. One of the 

 more developed crystals then appears as follows, 

 shewing the icositetrahedron — the ni Om of Naumann, 

 the specific variety of the crystal being apparently 

 2O2. Fig. 126. 



The colour also appears more distinct, and suggests 

 a sulistance used in some descriptions (f jewellery 

 known as cinnamon stone or essonite. The essonites 

 belong to the garnet family, which is divided into 

 many sections ; the essential constituent of all, with 

 i^^, if any exceptions, being silicate of alumina. 

 This composition, is accompanied liy certain oxides, 

 which, in different specimens, determine the class to 

 which the various garnets belong. Iron is, of course, 

 present in varying proportions, and in essonite, is 

 believed to impart the colouring matter. Lime is 

 another constant element which has caused essonite 



(and also another variety known as grossularite) to be 

 classed by Naumann and others as lime-alumina 

 garnets. Essonite appears to be widely distributed, 

 as it is found abundantly in the United Kingdom and 

 also in different localities in Europe and the United 

 States. In a good specimen in the Museum of the 

 College of Science here, the following combination 

 of the rhombic dodecahedron with the icositetra- 

 hedron (c^ O. Ill O m) is well developed. Fig. 127. 



Crossing over the top of the hill, the shore is 

 reached by a steep road. At the base of the hill 

 and stretching away to the right as far as Bray, is 

 a wide margin of strand, sloping gently to the water's 

 edge. Here the observer cannot fail to be struck 

 with the manner in which the flowing and ebbing 

 tides have sorted the sands into different sizes, from 

 coarse shingle to the very finest particles. Picking 

 up a handful at random, it is not difficult to distinguish 

 at least half-a-dozen distinct minerals, besides many 

 others, the exact nature of which requires further in- 

 vestigation. A pocket magnet reveals the presence 

 of quantities of magnetic iron, some of it being in 

 small lumps, quite large enough to enable the density 

 to be ascertained, and also to give the characteristic 

 black streak on a piece of porcelain. Filling my bag 

 with a sufficient quantity of the sand, I washed it on 

 my return home. This is an interesting process, and 

 for the benefit of any readers who may not be ac- 

 quainted with mineral work, I shall describe it. A 

 circular wooden dish is used for the purpose. It 

 should be of hard wood bevelled smoothly from the 

 rim to the centre, and not more than ten inches in 

 diameter for convenient use. Portions of the sand are 

 placed in this with a little water, and after a rotatory 

 movement, the water with the lighter particles of the 

 sand is poured off, leaving the heavier matter behind, 

 which then may be examined as desired. A small 

 portion being put into a test tube and treated with 

 strong hydrochloric acid, effervescence shews the 

 presence of limestone, whilst the yellow colour of the 

 solution indicates that iron in combination is being 

 dissolved. In all sands, particularly those in the 

 vicinity of the primitive rocks or their derivatives, 

 siliceous particles, such as pieces of quartz and felspar 

 constitute by far the largest portion. Proportions, of 

 course, vary according to localities, but there will 

 hardly be much error in stating, that, in the sand 

 under examination, these minerals reached at least 

 70 per cent. When the sand has been carefully sized 

 by passing it through a fine sieve beforehand, nearly 

 all the minerals whose specific gravity only reaches 

 about two, or a little beyond, pass ofT, leaving the 

 denser bodies behind. Amongst these latter we find 

 a large number of the essonite stones before described 

 disengaged from the matrix in which they were 

 formed, and now appearing in their perfect crystalline 

 form. Their specific gravity is about 3"5, and under 

 the blow-pipe, reactions both for iron and manganese 

 are obtained, but that for alumina is masked, owing 



