152 



GEOLOGY 



generally bake and alter overlying as well as 

 underlying rocks thus clearly indicating their 

 subsequent origin. Dykes (q.v.) consist generally 

 of crystalline rock which has been erupted in 

 approximately vertical and even-sided fissures, thus 

 giving rise to wall-like intnisions. Occasionally 

 fragmental igneous rocks, such as agglomerate, are 

 met with in similar positions. Veins is the term 

 applied to smaller irregular and more or less tortu- 

 ous intrusions of crystalline rock. Bosses (see 

 NECK) are amorphous masses of crystalline rock, 

 rising more or less vertically through surrounding 

 rock-masses. There is reason to believe that many 

 of these 'bosses' are the deep-seated reservoirs 

 from which volcanoes were supplied with lava. 

 'Dykes,' 'veins,' and sometimes 'sheets' proceed 

 from them into the adjacent rocks, which are often 

 much altered and metamorphosed. 



Structure of Derivative Rocks. The most char- 

 acteristic feature of these rocks is their bedding or 

 stratification a structure which is due to the mode 

 of their accumulation. Hence they are often spoken 

 of as the 'stratified rocks.' But, as we have seen, 

 stratification likewise characterises contemporane- 

 ous eruptive rocks. As far the larger number of 

 derivative rocks are simply aqueous mechanical and 

 chemical sediments, they are also often termed 

 'aqueous' and 'sedimentary rocks.' Individual 

 beds in a group of strata are lenticular or wedge- 

 shaped ; so that when any particular stratum is 

 followed in one direction it eventually thins away 

 and dies out. And the same is the case with 

 groups of strata. Fine-grained deposits such as 

 shale and limestone tend to be more persistent 

 and to cover wider areas than sandstones and 

 conglomerates. Almost any diversity of strata 

 may occur in a group or series, but it is more 

 usual to find certain kinds of rock associated 

 together ; thus, fine sandstone alternates with shale, 

 conglomerate with grit, limestone with fine shales, 

 &c. Again, individual beds are often found to 

 change their character as they are followed in 

 certain directions. Conglomerate, for example, 

 passes laterally into sandstone, sandstone becomes 

 argillaceous and passes into shale, while shale, by 

 the gradual increase of calcareous matter, becomes 

 marly and often passes into limestone. Sometimes 

 the stratification is extremely regular, at other 

 times the beds thicken and thin out very irregu- 

 larly, and not infrequently they show what is 

 called false-bedding^ ox current-bedding a structure 

 which is seen both in aqueous and eolian accumula- 

 tions ( see DUNES ). Amongst the surface-markings 

 seen in sedimentary rocks the most common are 

 ripple-marks, sun-cracks, rain-prints, and tracks, 

 trails, burrows, &c. of worms, crustaceans, mol- 

 luscs, reptiles, birds, &c. 



Strata are not often quite horizontal ; they 

 usually dip at a less or greater angle, and 

 such inclined strata are as a rule the remaining 

 portions of large curves or undulations, the upper 

 portions of which have been removed by denuda- 

 tion, so that the truncated strata crop out at the 

 surface (see OUTCROP, STRIKE). The simplest 

 form of curve assumed by a stratum is a mono- 

 cline, but anticlinal and synclinal folds occur 

 much more frequently (see ANTICLINE). In strata 

 with a moderate dip the strata on opposite sides 

 of an anticlinal axis incline at approximately the 

 same angle. But in more steeply inclined beds the 

 dip is often greater on one side than the other, 

 the beds on the steeper side of the fold becoming 

 doubled in below their equivalents on the other 

 side. This is what is termed ' Inversion ' a struc- 

 ture which when repeated gives us what are 

 called 'Isoclinal Folds' (see MOUNTAINS). In 

 regions of highly folded strata the fossils and even 

 the stones in conglomerates are often flattened 



and squeezed out of shape. Such deformation 

 likewise characterises whole rock-masses, as is well 

 seen in the structure termed Slaty Cleavage (see 

 CLEAVAG.E). As an extreme result of enormous 

 pressure we occasionally find that clastic rocks 

 have been converted into crystalline schists. 



Most rocks, as well igneous as derivative, become 

 gradually more and more consolidated. Soft inco- 

 herent sands and clays are compressed ; lavas cool 

 and harden. All rocks therefore tend to contract, 

 and in doing so they become cracked, regularly or 

 irregularly as the case may be. During the pro- 

 cess of folding they have likewise yielded to stress 

 and strain by cracking across. Such cracks are 

 termed Joints (q.v.). But rocks are not only 

 jointed ; frequently they are traversed by great, 

 fissures of displacement called Faults or Dislocations 

 (q.v.), which may sometimes be traced across the 

 whole breadth of a country. That the phenomena 

 of folding, fracturing, and displacement are the 

 result of earth-movements cannot be doubted, and 

 there is abundant evidence to show that such dis- 

 turbances have taken place again and again, some- 

 times over limited regions, at other times over very 

 much wider areas. This is proved by the pheno- 

 mena of Unconformity (q.v.), in which one set of 

 beds rests on the upturned and denuded ends of an 

 older series. 



The fissures and cavities of rocks are in some 

 places filled up again by the introduction of various 

 kinds of mineral matter through the chemical action 

 of percolating water. In many cases such mineral 

 deposition may have taken place from heated solu- 

 tions, under great pressure, and at great depths from 

 the surface. This is probably the origin of many 

 of the Ore-deposits (q.v.) met with as lodes or 

 veins. 



PAL.IEONTOLOGICAL GEOLOGY. A study of the 

 physical characters of rocks enables the geologist to 

 arrive at many interesting conclusions as to the 

 I mode in which rocks have originated. By such 

 I evidence alone it is sometimes possible to discover 

 the successive changes which some particular region 

 has undergone. Thus, the phenomena of igneous 

 and glacial accumulations tell their own story, and 

 even in the case of many sedimentary deposits 

 geologists are able, without the aid of fossils, to 

 distinguish between deep-sea and shallow-water 

 strata ; while certain rock-structures, such as un- 

 conformity, yield him evidence of changing physical 

 conditions. Without fossils, however, investiga- 

 tions into the successive phases through which the 

 earth's surface has passed could not proceed far : 

 historical geology would be impossible. It is 

 chiefly by means of Fossils (q.v.) that the deep-sea 

 or shallow-water origin and the marine or fresh- 

 water character of strata are determined, and the 

 climatic conditions under which they were deposited 

 are ascertained. When we learn that many fossils 

 belong to extinct species and even genera, and 

 that different groups of fossils occur in different 

 series of strata, it might seem, at first, as if this 

 would tend rather to confuse than aid the geolo- 

 gist. But the cause of such apparent discrepancies 

 lies, of course, in the simple fact that the fossili- 

 ferous strata belong to different ages some are 

 much older than others. In the uppermost or 

 youngest series the organic remains approach most 

 nearly to the life-forms of the present day, while 

 in the lower and therefore older strata the fossils 

 recede farther and farther from existing types as 

 we follow them to lower and lower geological hori- 

 zons. From this it would appear that there has 

 been a gradual coming-in and dying-out of species, 

 and observation has shown that when a particular 

 flora or fauna has died out it never reappears in 

 younger strata. When William Smith discovered 

 that each well-marked group of strata was charac- 



