In chemical composition the igneous rocks show wide variation• though limits are set by the laws of formation of magmas. No such limits are sec for the elastic rocks. By processes of alteration the igneous rocks change into the metamorphic rocks (as do also the elastic rocks), so that no sharp line can be drawn to separate these great ins.
great diflimilties arise in the systematie classi fication of ig-neous rocks, by reason of the many ways in which it is possible to describe them. and the varying importance NN 111(11 is attached to each method; hence great ditn.n.11..i. of 0111111011 exists among petrographers as to Hie hest scheme to be adopted. lIn the Continent of Europe, and espe cially in (;erinany. the manner of occurrence of a rock, whether as a boss or batholite (9.v•), as a dike (q.v.), or as a sheet or flow, is given the first importance; and the age of the rock, mice considered paramount, still retains its importance in the secondary classification. In _kilo-rico iipin ion se. nes to favor as bases of classification the chemical ecatiposition. the texture, and the min eral composition; but the future must be awaited before any (classification free from serious objec tions is likely to he adopted. The almost hope less confusion of the now in use is gen erally recognized. The reader is referred to the following works. in which are given detailed de scriptions of igneous rocks. Consult: Kemp, Handbook of Pocks (New York, I Ffpit) ; Rosen 1/ikroskopisebe asioyraph i,• (1, r 1/ i n( rol s and (Stuttgart, Itittin ; Zirkel, Lehrbuch d,r l'clroyrophie ( Leipzig. 18(4) Teall. British 1', 1 royra !dry (London, 18SS). tee (;Eof.m.y.
ICr'NIS FATUUS, li'it'f1-fis (Lat., vain fire). A luminous appearance frequently seen in marshy places, in churchyards, and over stagnant pools. It generally appears a little after sun set as a pale, bluish-colored flame, varying in size and shape; sometimes it shines steadily till morning, at other times disappears. nod reap
pears within abmit half-hourly intervals. It floats in air at about two feet from the ground, is sonietino-s fixed, and sometimes travels with great rapidity. :Many efforts have been made to discover its elms..., hut so varied are its appear 01111 so void of any eommon principle. that these attempts have totally failed. Two of the various explanations otTered may be mentioned here. The first is that the ignis fatmis is due to phosphureted hydrogen gas (P11,1, which pis the property of when it comes in contact with dry atmospheric. air: the gas wi old be generated by the decomposition of ani mal nettter present ill a marshy soil. The second is that it is dile to the combustion of methane, or marsh -gas. (11,. produced by the decomposition of vegetable Ratter; hut though this supposition aceolints for tinny appearances eonneeted with the ignis Intuits. the gas itself is not. spontaneously combustible. and an addi tional supposition becomes necessary to account for its ignition. The probable conclusion is that a number of phenomena, apparently similar, but arising from different causes, are aggregated tinder the term i?mis fattens.
The appearance of ignis fatuns is not a common phenomenon. and many distinguished naturalists who desired to inve‘tigate it have never suc ceeded in finding it hut it is not unfrequently seen in the north of Germany, the swamp and moorland districts in the south and northwest of England. and in the lowlands of Scotland. It is seen in the above pinoes from the middle of autumn till the beginning of November. Tn former times the ignis fatuus, under the names of Wilho'•the-NVisp, Jaek-a-lantern, Spunkie, etc., was an object of superstition among the inhabi tants of the districts where it appears, :Ind was believed to be due to the agency of evil spirits attempting to lore tra‘elers to their destruction.