Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/253

Rh iii. DYN.A.iiic'.iL.] smooth texture, and either dull white or pale grey, their fracture only slightly conclioidal, rarely splintery. 00:0 is a mud of organic origin found covering vast areas of the ﬂoor of the Atlantic and other oceans. Some of it is calcareous and formed wholly or mostly of the remains of 1v'oranL2'n1,'fcm, particularly of forms of the genus Gl0biger- Ina; hence this deposit has been termed foraminiferal or globigerina ooze. Sometimes it is mainly siliceous, consisting of the remains of ltrnliolarizc (lladiolarian ooze) or of diatoms (Diatom ooze). These de- posits are further referred to in the section of this article which treats of the geologiual aspects of the ocean. Shell-sand is a sand com- posed in great measure or wholly of comminuted shells, found com- monly on a low shelving coast exposed to prevalent on-shore winds. 'l‘his deposit when thrown above the reach of the waves and often wutted by rain, or by ti'iekling runnels of water, is apt to become - onsolidated into a mass, owing to the solution and redeposit of lime round the grains of shell. Flint and C’/wrt are siliceous rocks (which, though not strictly fragmental, may be conveniently placed here) found in nodules and layers in limestones of many different geological ages. Flint is a dark horny substance, breaking with a splintery to eoiielioidal fracture. It is p-.irtieularly abundant in the chalk formation. Chert is an impure ﬂint, containing more elay or lime with the silica. These substan :es seem in some cases to have had a directly organic origin, having been secreted from sea-water by the living organisms ; in other cases, where for example we ﬁnd a calcareous shell, or e.’-hinus, or coral, converted into silica, it would seem that the sub- stitution of silica for lime has been effected by a process of chemical pseudo:uorpliism either after or during the formation of the lime- stone.‘ (6.) Volcazzir: 1"ra_r/nmztal Ifocl.-s form an interesting group composed of the loose materials ejected from volcanic vents. In their typical condition they consist merely of consolidated volcanic debris, including bombs, scoriae, ejected blocks, sand, lapilli, and dust. lt is evident, however, that, when these materials were deposited in water, there would neces- sarily be a limit beyond which they would not extend, and where they would be mingled with and would insensibly pass into ordinary iioii-volcanic sediment. Hence we may expect to ﬁnd transitional varieties between rocks formed directly from the results of volcanic explosion and ordinary .-ediineiitary deposits. Moreover, as these fragmental vol- mnic masses usually consist almost wholly of the detritus' of different lavas, which have been blown into fragments in the volcanic chimneys, we may expect to ﬁnd, on the other hand, a passage from them into rocks derived from consolidated lava-beds by ordinary aqueous erosion. (See part iv.) Volcanic Cong//ouicratc is a rock composed mainly or wholly of rounded or subangular fragments cf any volcanic rocks in a. paste derived ehieﬂy or wholly from the same materials, usually exhibiting a stratified arrangement, and often found intercalated between suc- cessive sheets of lava. In most cases conglomerates of this kind have been formed by the accumulation of materials ejected from vol- canic vents; occasionally, as just remarked, they may have resulted from the aqueous erosion of previously, solidiﬁed lavas, or from a combination of both these processes. There does not appear at pre.sent to be any satisfactory method of always determining the exact mode of formation, except that well-rounded and smoothed stones will almost certainly indicate long-continued water-action rather than trituration in a volcanic vent. The volcanic conglomerates may receive different names according to the nature of the component fragments: thus we have basalt-cow _r/lonzcratcs, where these fragments are wholly or mainly of basalt, Irt.ch;/lc-canglonmulcs, porpli3/rite-conglomerates, phonolz'tc-co7iglo- uicxates, &e. Volccmie Brcccia resembles volcanic conglomerates, except that the stones are angular. This angularity indicates an absence of aqueous erosion, and, under the circumstances in which it is found, usually points to volcanic explosions. There is a great variety of breccias, as ba.s-alt-brcccia, d-iabase-brcccia, &c. Volcanic Agglomcratc is the name given to a tumultuous assem- blage of blocks of all sizes up to masses several yards in dia- meter. It is met with in the “necks” or pipes of old volcanic ori- ﬁces. The stones and paste are commonly of one or more volcanic rocks, such as basalt or porphyrite, but they include also fragments of the surrounding rocks, whatever these may be, through which the volcanic oriﬁce has been drilled. As a rule agglomcrate is devoid of stratification; but sometimes it includes portions which have a more or less distinct arrangement in beds of coarser and ﬁner detritus, often placed on end or inclined in different directions at high angles. 1 Hull and Hardman on Chert, Trans. Roy. Dub. Soc., new ser., vol. i. 71, 1878. GEOLOGY 239 Volccmic Tuﬁ'.—Tliis general term may be made to include all the finer kinds of volcanic detritus, ranging on the one hand through coarse gravelly deposits into conglomerates, and on the other into exceedingly compact ﬁne-grained rocks formed of the ﬁnest and most impalpablc kind of volcanic dust. Some tuffs are full of microlites or imperfect forms of crystallization derived from the lava which has blown into dust. Others are formed of small rounded or angular grains of diﬂ"erent lavas with fragments of various rocks through which the volcanic funnels have been drilled. Minutely cellular grains, as if derived from the ebullition of very ﬂuid glassy lava like palagoiiite, constitute much of the tuff in some of the volcanic necks of Carboniferous age in central Scotland. Some tulfs have consolidated under water, others on dry land. As a rule they are distinctly stratified. Near the original vents of eruption they commonly present rapid alternations of ﬁner and coarser detritus indicative of successive phases of volcanic activity. The tuffs may be subdivided according to the nature of the lava from the disintegration of which they have been formed. Thus we have fclsitc-tzcﬂs, trachytc-tuﬁ3', basalt-tuﬂs, pzmiice-t1(‘.ﬁZs, p07'phyn'lc- tuﬁs-, palagonitc-tuﬁc. Some varieties have received special names. Truss (Duckstcin, Tuﬂs-tcin) is a compact yellow pumieeous tuﬂ which has ﬁlled up some of the valleys of the Eifel region and is lar ely quarried as an hydraulic mortar. Pcpcri-no is an Italian tuff of ate geological date, full of separate crystals of augitc and other minerals. PART IlI.—DYNAMlOAL GEOLOGY. Under this section is included the investigation of those processes of change which are at present in progress upon the earth, whereby modiﬁcations are made on the structure and composition of the crust, on the relations between the interior and the surface, as shown by volcanoes, earthquakes, and other terrestrial disturbances, on the distribution of oceans and continents, on the outlines of the land, on the form and depth of the sea—bottoin, on climate, and on the races of plants and animals by which the earth is tenanted. It brings before us, in short, the whole range of activities which it is the province of geology to study, and leads us to precise notions regarding their relations to each other, and the results which they achieve. A knowledge of this branch of the subject is thus the essential groundwork of a true and fruitful acquaintance with the principles of geology, seeing that it necessitates a study of the present order of nature, and thus provides a key for the interpretation of the past. The whole range of operations included within the scope of inquiry in this branch of the science may be regarded as a vast cycle of change, into which we may break at any point, and round which we may travel, only to ﬁnd our- selves‘ brought back to our starting-point. It is a matter of comparatively small moment at what part of the cycle we begin our inquiries. VVe shall always ﬁnd that the changes we see in action have resulted from some that preceded, and give place to others which follow them. At an early time in the eartli’s history, anterior to any of the periods of which a record remains in the visible rocks, the chief sources of geological action probably lay within the earth itself. The planet still retained a great store of its initial heat, and in all likelihood was the theatre of great chemical changes, giving rise, perhaps, to manifesta- tions of volcanic energy somewhat like those which have so marvellously roughened the surface of the moon. As the outer layers of the globe cooled, and the disturbances due to internal heat and chemical action became less marked, the inﬂuence of the sun, which must always have operated, would then stand out more clearly, giving rise to that wide circle of superﬁcial changes wherein variations of tempera- ture and the circulation of air and water over the surface of the earth come into play. ' In the pursuit of his inquiries into the" past history and into the present régime of the earth, the geologist. must needs keep his mind ever open to the reception of evidence for kinds and especially for degrees of action which he had not before imagined. Human experience has been too short