Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/250

Rh 236 lamination ; (2) that of cleavage. as in clay slat.e ; (3) that of foliation, as in the schists. There is a fourth kind of divisional planes, that of joints, sometimes so closely placed together as almost to rival the others, as will be pointed out in part iv. .11 ica-schist (.l[ {ca-slate) is a schistose aggregate of quartz and mica. The relative proportions of the two minerals vary widely even in the same mass of rock. Each is arranged in lcntieular wavy lamime. The quartz shows greater inconstancy in the number and thickness of its folia. Frequently a layer of this mineral may often be seen to swell out to a thickness of an inch or more, and, dwindling rapidly down to a mere thread, disappear. The quartz may often be ob- served to rctai11 a nular character like that of quartz-rock, no doubt indicative oglilts originally sedimentary origin (see fig. 6). The mica lies in thin plates, sometimes so dovetailed into each other as to form long continuous irregular crumpled folia, separating the quartz layers, and often in the form of thin spangles and mem- branes running in the quartz. Among the accessory minerals, ar11et, felspar, and hornblende are not infrequent. lIica-schist orms extensive regions in Norway, Scotland, the Alps, and other parts of Europe, in connexion with other members of the schistose family of rocks. It is also found encircling granite masses in Scot- land and Ireland as a metamorphic zone a mile or so broad, which shades away into the unaltered strata of greywaeke or slatc outside. Though the possession of a ﬁssile structure, showing abundant divisional surfaces covered with glistening mica, is characteristic of mica-schist, we must distinguish between this structure and that of many micaceous sandstones which can be split into thin seams each splendent with the sheen of its mica-flakes. A little examination will show that in the latter case the mica has not crystallized in sitw, but exists merely in the form of detached worn scales, which, though lying on the same general plain, are not welded into each other as in a schist; also that the quartz does not exist in folia but in rounded separate grains. Gnciss is a crystalline schistose aggregate of the same minerals as in granite—felspar, quartz, and mica. The relative proportions of these minerals, and the manner in which they are grouped with each other, give rise to numerous varieties of the rock. As a rule the folia are coarser and the schistose character less perfect than in mica-schist. Sometimes the quartz lies in tolerably pure bands a foot or even more in thickness with plates of mica scattered through GEOLOGY [IL GEOGNOSY. of quartz and felspar, or the felspar will take the form of independent lenticular folia, while the mica runs abundantly through the rock, and by its own rcady cleavage imparts a ﬁssile structure to the whole. Sometimes the mica is mostly or wholly replaced by hornblende, in other cases by talc (1n'otogz'nc-gzwiss). Like mica-schist, gneiss occupies a large space in regions where the older geological forma- tions come to the surface. Varieties of it are also found in the metamorphic zone encircling some masses of granite. So coarse is the texture of many gncisses that they cannot, in hand-specimens nor even in large blocks, be certainly discriminated from granite. In such cases it is only by examination in the ﬁeld and the detection of clear evidence of foliatcd structure that their true character can be determined. An interesting and important variety is mct with in some regions of gneiss and schist, viz. , conglomerate bands in which pebbles of quartz and other materials from less than an inch to more than a foot in diameter are imbeddcd ill a foliatcd matrix. Examples of this kind are found in the pass of the Téte N oir between Martigny and Chamouni, i11 N. W. Ireland, in the islands of Bntc and ls-lay, and in different parts of Argyllshirc. These cnelosures are not to be dis- tinguished from the ordinary watcr-worn blocks of true conglomer- ates; but the original matrix which encloses them has been so altered as to acquire a mieaeeous foliatcd structure, and to wrap the pebbles round as with a kind of glaze. These facts are of considerable value in regard to the theory of the origin of the crystalline schists. Gramclilc (Lcptynitc) is a crystalline schistose aggregate of orthoclase and quartz, with some garnet and kyanite. Clzloritc-schist (or Chlo-rz'tc-slalc) is a schistose aggregate of green ehlorite, often with some quartz, felspar, mica, or tale. The more massive forms (lapis ollm-is, potstone) can be cut as building stone, or for the manufacture of articles for domestic use. Talc-schist is a schistose aggregate of whitish-green or yellowish talc often combined with fclspar or quartz. Dr Ileddlc has recently shown that many so—callcd talc-schists contain no tale, but own- their nnetuous character to a variety of mica (margarodite). I10rnblc7Ldc-schist is a schistose mass of black or dark—grecn hm n- blcnde, but often interleaved with felspar, quartz, or mica. Vlu-n the schistose character disappears, the mass becomes a Izornblrm/r -rock (amphibolite). When the variety actinolite occurs instead of common hornblende it forms actinolite-schist. Numerous other varieties of schists have been described, but they occupy very subordinate places among the foliatcd rocks. The following analyses show the chemical Composition of the more it. These quartz layers may be replaced by a crystalline mixture important of those which have been enumerated 2- . Protoxide i _ _. Silica. Alumina. 3xI'S:]S_ (Eta Magnesia. Lime. Soda. Potash. Water. Mica-schist ................... .. 65'13 l8'l6 5'27 0'51 2'70 0'32 0 '53 2'99 3'73 TiO.,, 1 '54 = 100'88 277:3, l Mica gneiss ................... .. 70'20 14'04 6'84 0'80 2'03 0'91 2'98 1'67 ,, 0'72 =100'_19 | Hornblende gneiss .......... .. 56'83 19'68 8'64 trace 3-28 1-89 3-14 2-34 2-69 C30 3:3; = 99-05 2-80 I Granulite ..................... .. 73'47 14-'86 3'28 .. . 0'67 1 '62 I '80 3'95 0 '57 .. . =- l 00'22  I Chlorite-schist .............. .. 42'08 3 '5] 26'85 0'59 17'l0 1 '04  11 '24 = 102 '41 2'75 I Tale-schist ................... .. 50-81 4-53 7-58 31-55    4-42 = 98-89  [ Hornblende-rock ........... .. 49 '42 18'12 15'01 3'16 8'65 2'57 1'27 1 '80 loss =10!) '00 As the disappearance of the schistose structure produces a crystalline amorphous compound like that of a massive or ordinary igneous rock, we are brought at last round again to rocks which we cannot distinguish from those to which elsewhere an igneous origin is assigned. In gneiss, for example, the same minerals occur which form granite, and possess a crystalline character. Any process, such as irregular internal motion of the mass, which could destroy the schistose structure and produce a thoroughly granite-like texture, would give rise to a rock which, what- ever its previous history might have been, could not be dis- tinguished from granite. That such internal transforma- tions have taken place among the crystalline gneissose masses can hardly be doubted. And thus, at the one end of the schistose series, we may have ordinary unaltered sedi- ment; at the other, after many intermediate stages, a thoroughly crystalline amorphous rock like granite or syenite. II. Fragniental (Clastic) Rocks. This great series embraces all rocks of a secondary or derivative origin; in other words, all formed of par-, ticles which had previously existed on the surface of the earth in another form, and the accumulation and consoli- dation of which gave rise to new compounds. Some of these rocks have been produced by the mechanical action of running water, such as gravel, sand, and mud; others have arisen from the gathering together of the remains of once living plants or animals; others have been formed by the consolidation of the loose debris thrown out by volcanoes. ((5.) Gravel (mcl Sand Ia’0cK'.s'.———Or(linary gravel and saw! are produced by the action of running water on every sca- coast and river-course. These sedimentary materials, being mere mechanical formations, vary indeﬁnitelyin composition, according to the nature of the source from which they are derived. As a rule they consist of the detritus of siliceous rocks, these being among the most durable [1lalCi‘l:1l$‘. Quartz, in particular, enters largely into the composition of sandy and gravelly detritus. Fragmentary materials tend to group themselves according to their size and relative density. Hence they are apt to occur in layers, and to show the I characteristic slmlz_'/‘icrl arrangement of se«Iimenta7'_2/ rocks. They may enclose the remains of any plants or animals entombed on the same sea-ﬂoor, river-bed, or lake.-bottom.