170 



KNOWLEDGE 



[August 1, 1900. 



many examples have been recorded. Its interest lies 

 chiefly in the difficulty of finding a satisfactory explana- 

 tion, or rather in deciding which of three or four 

 possible explanations is the true one in any particular 

 case. The numerous observations which Mr. Oldham 

 has collected show that, during the Indian earthquake, 

 neighbouring objects similarly placed were generally, 

 but not always, 'twisted in the same direction; and he 

 adopts the view, at which, however, he arrived inde- 

 pendently, that rotation is chiefly due to changes in the 

 direction of the shock. The detached part of the 

 pillar, he believes, is tilted on one edge, and then, 

 before it has ceased to rock, is twisted about that edge 

 by later movements taking place in different directions. 



Structure of the Epicentral District. 

 A large part of the epicentral district is situated in 

 a group of hills lying to the south of the Brahmaputra 

 valley, to which the name of the Assam Range has been 

 given. " It is an elevated tract composed of crystalline 

 gneissic and granitic rocks, with some metamorphic 

 schists and quartzite, which carries a varying thickness 

 of cretaceovis and tertiary rocks along its southern 

 edge." Mr. Oldham distinguishes three stages in the 

 history of the range. There was first an old land- 

 surface which, in course of time, was worn down by 

 rain and rivers till they almost ceased to affect its 

 form. Traces of this surface are still visible in the 

 plateau character of the mass. It was then elevated, 

 not uniformly, but along a series of faults, so that it 

 consists now of a succession of ranges, the face of each 

 range being a fault-scarp, and its crest the edge of an 

 adjoining plateau sloping away from the summit. With 

 this elevation began the third and last stage. The 

 streams were able to work again, and deep gorges were 

 carved out of the range, so far that in parts its original 

 character is nearly effaced. But the retention of that 

 character in other districts is of course evidence of the 

 comparatively recent period of the final elevation. 



Permanent Changes in the Epicentral Area. 



Faults and fractures in the earth's crust are among 

 the most remarkable of these disturbances. They are 

 quite distinct from the fissures which occur m 

 alluvial ground. The former are of deep-seated, the 

 latter of superficial, origin ; the one are connected with 

 the causes of the earthquake, the other are merely its 

 effects. The longest of these faults was traced by Mr. 

 Oldham in the Chedrang valley (about 35 miles north- 

 east of Tura) for a distance of twelve miles or more. 

 Running in a nearly straight path from S.S.E. to 

 N.N.W., the fault is crossed about a dozen times by 

 the river, which at these points is either broken into 

 waterfalls or ponded back by the vertical face of the 

 fault. Pools of some extent are also formed by the 

 blocking of the drainage in the western tributarv 

 valleys; for, wherever a change of level is perceptible, 

 it is always the rock on the east side of the fault that 

 has been elevated with respect to the other. The throw, 

 or amount of elevation, varies considerably; the highest 

 measured being 35 feet. In two places, it falls as low 

 as zero; and here are formed broad sheets of water 

 chiefly on the eastern side of the fault, and blocked, 

 not by the fault-scarp itself, but by the undulation in 

 the surface of the ground due to the increase of throw 

 further down the valley. 



Another fault-scarp, described by Mr. Oldham, is 'Ih 

 miles in length, with a maximum throw of 10 feet. There 

 are also fractures along which the throw is either very 

 small or imperceptible. The largest of these is the BorJ- 



war fracture, about fifty miles east of the Chedrang fault. 

 Near Bordwar, it crosses a low hill of gneiss, which it 

 has rent in two. In the immediate neighbourhood of 

 the fracture, the violence of the shock was extreme. 

 Trees were overthrown or killed as they stood, and huge 

 masses of rock were rolled down the slope. When the 

 hill is left, the course of the fracture can be followed 

 for a total length of about seven miles, being marked 

 by landslips or by bands along which trees have been 

 snapped across or overthrown. 



While the crust was thus fractured without per- 

 ceptible change of level, it was, in other places, thrown 

 into long low folds which are apparently independent 

 of faults. These are most easily detected when they 

 cross the beds of rivers and are sufficient to reverse the 

 direction of the drainage. There are then formed 

 small lakes or pools, like the two which occur on the 

 east side of the Chedrang fault. About 15 or 20 miles 

 to the south of this fault, there is a group of such pools, 

 a mile or more in length. The depth of the water in- 

 creases gradually from both ends, until it reaches from 

 10 to 18 feet, and here may be seen trees and clumps 

 of bamboos standing in the water and killed by the 

 immersion of their roots. 



There are, again, other facts which point to changes 

 of level having taken place over a wide area. From 

 Mao-phlang, near Shillong, a road leads to the 

 neighbouring station of Mairang. Before the earth- 

 quake, only a short stretch of this road could be seen, 

 where it rounde3 a spur at about three miles' distance. 

 Now, a much longer stretch is visible, and it can also be 

 seen passing round the next spur. From a road about 

 five miles from the southern end of the Chedrang fault, 

 it used to be only just possible to see the Brahmaputra 

 over an intei-vening hill ; now, the whole width of the 

 river has come into view. At Tura, which is 95 miles 

 west of Mao-phlang, a battalion of military police were 

 accustomed to signal by heliograph with another station, 

 Rowmari, 15 miles further to the west. This, formerly, 

 could just be done by means of a ray which grazed a 

 hill between the two places ; it can now be done quite 

 easily, and, in addition, a broad stretch of the plains 

 east of the Brahmaputra is visible from the same spot. 

 Thus, we see that the permanent changes have taken 

 place over the northern part of the Assam Hills for 

 a distance of about a hundred miles from east to west. 



During the cold weather of 1897-1898, a revision of 

 certain triangles was carried out by the Survey, but 

 they were limited to the eastern part of the epicentral 

 area, as the focus was at that time supposed to lie 

 under the Khasi Hills. Of the 16 sides, only one was 

 apparently unaltered in length, two were shortened by 

 an inch or two, while the others were all lengthened 

 by amounts varying from one to eight or nine feet. 

 The heights of most of the stations were also found to 

 be increased, one, close to a conspicuous fault-scarp, by 

 as much as 24 fact. Unfortunately, all of these figures 

 are rendered uncertain by the choice of the statiojs 

 which form the extremities of the new base-line. One of 

 them lies inside the epicentral area, and the other out- 

 side, the line joining them limning nearly north and 

 south. But, as compression in this direction is to be 

 expected, it is probable that this line has been shortened, 

 and the assumption that its length was unchanged would 

 therefore lead to an ajiparent expansion of all the other 

 .sides. The only result of the re-survey is thus to place 

 beyond doubt the fact that very important changes of 

 some kind have taken place since the survey was first 

 made in 1860. 



