997 



GEOLOGY. 



GEOPHILA. 



99 -i 



country should be known to the engineer, as to its mineral structure 

 as well as its elevation, in order that the situation of these may be 

 properly fixed. It was inconvenient to make the Thames and Severn 

 tunnel at its present level, often much above the level of the spring 

 which is called the source of the Thames, and in the thirsty oolitic 

 rocks ; for thus the cost of maintaining the supply of water by puddling 

 the canal, and engines for pumping, has been found very oppressive. 

 Tunnels and summit levels for canals should certainly be made in 

 argillaceous rocks, and geological investigations will often point out 

 situations where, from particular displacements of the rocks, this is 

 practicable, even in a range of hills BO continuous and so calcareous 

 as the chalk or the oolites. 



The same rules do not apply to railroads, which, on the contrary, 

 may often be beneficially carried through dry rocky hills which would 

 absorb all the water of a canal. 



In the execution of the works of canals and railroads, a good geolo- 

 gical map would often be found more serviceable as a guide to the 

 engineer than a great number of borings, unless these were placed in 

 situations corresponding to the variations of the strata, which such a, 

 map would indicate. 



In some favoured countries the labours of the sculptor and the 

 architect are scarcely injured by exposure to the atmosphere for 2000 

 : while in our damp and changeable climate even the interiors 

 of cathedrals show, by the decay of their marbles and the destruction 

 of the stone walls, the necessity for an architect to study the dura- 

 bility of his materials. It is remarkable that the Romans were 

 more prudent or more fortunate in their choice of stone for buildings 

 in Bath and York than their successors have been. The relics in the 

 Institution at Bath abundantly prove that the rag beds of the oolite 

 are more durable than the finer and handsomer freestone which the 

 enterprise of Allen first introduced to common use. The tnagnesian 

 limestone in the Roman walls of York is in far better condition of 

 preservation than most of that which is of only half the age in the 

 face of the cathedral. 



The Saxons in the north of England used the coarse and durable 

 n>illtoue-grit, which on the brows of the high mountains of Derby- 

 shire and Yorkshire stands conspicuous for its bold defiance to the 

 elements. In choosing from any given rock the parts which are most 

 fitted for permanent edifices, the examination of nature is perhaps 

 more instructive than even a study of buildings. Not every sort of 



water exists in the deeper parts of the earth, and in fact fills the 

 whole space left by fissures in the rocks, unless where, as in diagram, 

 Jiff. 10, there be a fault which breaks the continuity of the communi- 

 cations along the rocks. At the surface there will be generally one or 

 more springs (2) along the line of such fault, F. 



In sinking deep pits it is generally found that argillaceous strata 

 are quite dry within ; for example, in the diagram above referred to, 

 the well a, supposed to be sunk in the London clay, yields no water ; 

 but the other strata, alternating with the clays, yield water in greater 

 or less quantity, and of quality corresponding with the nature of the 

 rock. Thus the well b, sunk down to the sands, lignites, &c., of the 

 plastic clay, yields some water, not always of good quality ; but when 

 the well, as c, is made to reach to and penetrate the chalk, a great 

 body of good water commonly rises from that rock. [WATER ; ARTE- 

 SIAN WELL, in ARTS AND Sc. Div.] 



To drain land is to intercept the natural springs : this can never be 

 done upon good principles unless the geological structure of the dis- 

 trict be known. When porous rocks alternate with strata impervious 

 to water, the springs will commonly issue at several points on the 

 surface-line of junction of the strata, as at x and y in diagram, fig. 1 ; 

 and by making a deep drain along the Hue of junction, Dr. Smith has 

 often accomplished the complete desiccation of wet lands in the oolitic 

 districts of England, which had been, in vain guttered in all directions 

 by the usual hollow drains. 



The same principle applies, but not with the same ease of success, 

 to the draining of districts where gravel and clay are much inter- 

 mingled. The gravel acts as a porous rock, but its irregular distribu- 

 tion renders the operation of deep draining costly and less effectual. 



From the same principles it follows that springs may be regulated, 

 and the subterranean reservoirs employed to store up water in the 

 winter, when it is little wanted, for the purpose of supplying the 

 demand in summer. This has actually been done by Dr. W. Smith, 

 who opened, in the sandstone rocks near Scarborough, a subterranean 

 reservoir ou the site of a little spring, closed it with a dam, and regu- 

 lated the discharge for the benefit of the town. [SPRINGS.] 



(Lyell, Principles of Geology ; Lyell, Elementary Geology ; Ansted, 

 Geology, Introductory, Descriptive, and Practical; Ansted, Elementary 

 Course of Geology ; Phillips, Guide to Geology ; Jukes, Popular Physical 

 Geology ; De la Beche, How to Observe in Geology ; Portlock, A Rudi- 

 mentary Treatise on Geology.) 



a, b, r, wells ; L, London clay ; P, plastic clay and sands ; C, chalk ; g, ganlt; G, lower grecnsand ; W, wcalden ; i, y, is, springs ; the last at a firalt, F. 



granite resists the carbonic acid and moisture of the air ; but while 

 the rolled blocks from Snap-Fell retain, after thousands of years' expo- 

 sure on the surface, their surfaces of attrition, the granitic top of 

 Castle Abhol, in Arran, is so rotten that it may be easily beaten to 

 fragments by a hammer. The millstone-grit of Brimham is almost ; 

 I away over a hundred acres, while that of Agra Crags appears ; 

 to l>e more capable of withstanding the same agencies; and the 

 limidical stones of Boroughbridge have stood the storms of 2000 

 years, with little more injury than a few rain-channels which scarcely 

 reach the ground. 



To the agriculturist geology has rendered some services, and pro- 

 bably may in future be appealed to for further aid. Lister's proposal 

 for the construction of a map of soils was only partially executed, after 

 a century, in some of the county reports made to the Board of Agri- 

 culture. The principal use, as it appears to us, of such a map (aud 

 this is in fact supplied by the maps of strata), is to aid the statistics 

 of agriculture by furnishing a basis for comparing the agricultural 

 practices on similar and dissimilar soils. 



But geologieal science will appear more intimately connected with 

 agricultural improvements if we consider it as the basis of all sound 

 knowledge of springs and the subterranean distribution of water. 

 The rain which falls from the heavens upon all soils and rocks 

 in.lill'. rently, runs off the clays, but pinks into the limestones, sand- 

 utones, and other rocks, whose open joints act like so many hidden 

 ir. Owing to the complicated intercommunication of the 

 fissures, these reservoirs are slowly filled and slowly emptied ; both 

 the supply from rain and the discharge from springs may and gene- 

 rally do go on together ; and the jointed rocks may be viewed as 

 rijimliaing the supply and expenditure. 



lint below the level of the springs thus formed, a great body of 



GEOMALACUS (Allman) a genus of Molluscous Animals belonging 

 to the family Limacidce. [LlMACiD^;.] 



GEOMYS. [MmtiDjE.] 



GE'OPHILA (from yri, the earth, and <j>i\, love), a genus of Plants 

 belonging to the natural order Cinchonaceir. It has the limb of the 

 calyx 5-parted, with linear spreading segments ; the corolla tubular, 

 with a pilose throat and 5 rather recurved lobes, with 5 anthers 

 inclosed ; the stigma bifid ; the berry ovoid, angular, crowned by the 

 ; calyx, 2-celled, 2-seeded. 



The species are creeping herbaceous plants with stalked cordate 

 leaves, like those of a violet f the stipules are solitary, undivided ; 

 the flowers sub-sessile, umbellate, surrounded by bracts, which aro 

 shorter than the flowers. 



G. reniformis has the petioles hairy above ; reniform obtuse leaves, 

 with the lobes at the base approximate; the bracts linear; the 

 peduncles 4-C-flowered, shorter than the leaves. It is a native of 

 moist shady places in the hotter parts of America, as Havanua, 

 Jamaica, Puerto Rico, Brazil, and the basin of the Orinoco. The root 

 of this plant is emetic, and may be used with advantage as a substi- 

 tute for ipecacuanha. 



G. violacea has cordate reniform leaves, obtuse, glabrous, with the 

 lobes approximate at the base; petioles hairy above; umbels few- 

 flowered, almost sessile between the ultimate pair of leaves ; braets 

 linear-lanceolate. It is a native of Guyana, in woods, and of the 

 Isthmus of Panama. It diffi-rs from (r. reniformis by the petioles 

 being shorter, the umbels hardly pedunculate, the corollas violaceous, 

 and the berries blue. 



There are several other species of this genus, all of which were 

 formerly referred to the genus Psyckotria. They are G. diversifolia, 

 G. viola-folia, li. macropoda, and G. grucilis, 



