78 THOMAS STERRY HUNT ON A NATURAL SYSTEM IN 



first two tribes are unknown, or donljtfuUy represented, in the third suborder. The 

 adamautoids in the three suborders are distinguished by their hardness and their molecu- 

 lar condensation, and, save in the case of the more basic protadamantoids, by their 

 resistance to acids. A small proportion of water enters into the composition of many of 

 these species, not only in bertrandite, but in euclase and beryl, and even in epidote and 

 tourmaline, in worthite and in malacone. It is to be noted also that the aluminic proto- 

 peradamantoids are not, so far as is known, generated by cooling from igneous fusion ; Init, 

 on the contrary, by the action of heat, even below their melting points, undergo a chemi- 

 cal change, shown by a diminution of density and by a susceptibility to the action of acids 

 (§ 36). The atomic volume of the adamantoids, while in certain cases not very far removed 

 from that of si^athoids, is always less, and in the harder or more gem-like species indi- 

 cates a great degree of condensation. These characters are especially marked in the per- 

 adamantoids, which include the silicates of the lowest known atomic volumes. 



The great phylloid or micaceous type is, like the adamantoici type, represented in 

 each suborder, and approaches it in condensation. The phylloids in the second suborder, 

 w'here they are most largely developed, include both anhydrous and hydrated species, and 

 in the less protobasic forms exhibit much of the same chemical indifference to acids and to 

 atmospheric action which distinguishes the aluminic adamautoids. The species of all 

 these tribes are ci'ystalline, and the system of crystallization to which they belong has, 

 wherever known, been given in the preceding tables. 



§ 102. The colloid forms of matter which appear in each siiborder, and which w^e 

 have designated respectively ophitoids, pinitoids and argilloids, are, as is well known, 

 generated both by aqueous and igneous processes, and hence include, as might be ex- 

 pected, both hydrovis and anhydrous species. Those formed either directly or indirectly 

 by the igneous method are necessarily very indefinite in composition, being volcanic 

 glasses or the results of their hydration. The tendency to chemical change exhibited by 

 colloids was insisted on by Grraham, and in A'iew of this characteristic, as shown by 

 Bunsen in his studies of palagonite, which is readily transformed by heat, in part, into a 

 crystalline zeolite, the writer has elsewhere spoken of this hydrated protopercoUoid as 

 a mineral protoplasm,' a designation equally applicable to other colloidal silicates. Of 

 these, serpentine gives rise to various crystalline species, often hydrated, such as chrysotile, 

 marmolite, talc, enstatite and chrysolite, which are generated in its mass by aqueous 

 action, while by igneous fusion it is changed into a mixture of enstatite and chrysolite. 



§ 104. Obsidian and pitchstone with its varieties, and tachylite and palagonite, 

 are examples of protopercoUoid silicates more or less hydrated, with the exception of the 

 first, and susceptible, like artificial glasses, of devitrification or conversion into crystalline 

 silicates. This process is well illustrated by the studies of Fouqué and Michel Levy, who 

 have shown that by maintaining in "fusion for many hours vitreous mixtures of proper 

 composition, it is possible to produce at will such crystallized species as chrysolite, pyrox- 

 ene, magnetite, feldspar, leucite and melilite, which separate from the colloid mass, as 

 from a solution, at temperatures below the fusing points of these species.- Similar trans- 

 formations occur in the presence of water, as when common glass by the action of water at 



'See the Origin of Crystalline Rocks, Trans. Roy. Soc. Canada, Vol. ii. Sec. 3. pp. 33, fiS. 

 ^Syntlii'SO (les Mini'-ranx et des Roches, Fouqué et Michel Levy, 1882, pp. 45, 80. 



