« PreviousContinue »
formation of the gneissose granite where Kalogarhi now is, there would have been some phenomena of metamorphism exhibited by the tertiaries themselves, especially since in the Kotedwar glen they are only distant 5 or 6 miles from the intrusive boss of Kolagarhi.
On the whole, it would seem that Plutonic action first, followed by mechanical strain due to tangential pressure as the Himalaya continued and continue growing, must be considered the order in which the agents of change have acted on the Kalogarhi and Dudatoli areas. That this lateral pressure acting on slates and schists should bend them to its will, is not surprising; but we can also imagine that wherever they were stayed by ribs of massive granite, they would have strength to resist largely the contorting pressure, which would then spend itself producing rearrangements of the particles and of the crystalline texture of the rocks, thus imprinting on them a foliated or semi-foliated character.
REFERENCES TO PLATE.
Fig. 1. Garnet in schist, shewing mica films folding round it.
2. Gneissose granite, FOLIATED: tabular.
5. Quartz schist shewing lenticular-tabular foliation a=pebble.
6. Section at Marwara, near Kainur: a=gneissose granite mass. a' thin bands of gneissose granite b=mica-schist.
7. Inclusion of mica-schist (b) in gneissose granite (porphyritic "augen" near Kainur. a porphyritic_orthoclase crystal.
Preliminary Sketch of the Geology of Simla and Jutogh,1 by R. D. OLDHAM, A.R.S.M., Deputy Superintendent, Geological Survey of India. (With a map.)
The geological structure of the Simla hill station, if regarded in detail, is one of extreme complexity of varying dips and innumerable faults, but on the large scale it is on the contrary simple to a degree, as may be seen on the accompanying
The oldest rocks are exposed along eastern and north-eastern limits of the map and all along the road to Mashobra; they consist of fine-grained micaceous grey slates with occasional quartzite bands, originally described by Mr. Medlicott as the infra-Blaini or Simla slates. On the spurs either side of Annandale the prevailing dip is south-west-by-south which further east bends round to westsouth-west. Near the upper limit of these slates there are some bands of a very trap-like sandstone or grit which I have only seen on the spur between Annandale
1 The observations on which this account is based were made during a brief stay in Simla while preparing to start into the interior: the main features will be found fairly correct, but the boundaries of the rock groups have in many cases been sketched in without having been followed in detail.
and the glen and again near Sanjaoli, though it would probably be found in a corresponding position elsewhere. When weathered it is impossible to tell this rock from a decomposed trap, and even the unweathered rock has a very trap-like appearance, but its detrital origin is shown by the presence of rugged scales of mica arranged parallel with the bedding.
At their upper limit these beds become impregnated with carbonaceous matter, and in consequence the colour becomes darker and the streak changes from pale grey to black. They are succeeded by a remarkable and important set of beds, called the Blini (since changed to Blaini) by Mr. Medlicott, from their occurrence in the valley of the Blaini stream hich flows westwards from Solon.
The group was described by Mr. Medlicott as consisting of a pure hard limestone, generally pink, composed of thin beds aggregating 15 to 20 feet thick, and a conglomerate consisting of a matrix of fine-grained slates through which pebbles of quartz or slate were scattered. As applied to the group in the Simla neighbourhood, this description must be amplified, for, underlying the limestone and "conglomerate," there come carbonaceous slates, which usually weather white on the bedding surfaces, and below these comes another "conglomerate," the whole being probably about 200 feet thick.
As this group is one of great interest and, owing to its marked characteristics, great importance, it will be well to clear the ground by explaining that what has been called a conglomerate in the foregoing passage is not a conglomerate at all in the usual acceptation of the term. The rock consists of a fine-grained matrix through which are scattered blocks of slate and quartzite, as a rule angular or subangular, in some cases containing 8 or 10 cubic feet of stone. It is evident that a current sufficiently violent to move these blocks would have swept away the fine silt in which they were imbedded, while, per contra, any current sluggish enough to allow of the deposition of this silt would be impotent to move the small pebbles, to say nothing of the larger blocks which the now indurated silt encloses. The only possible explanation is that these fragments were dropped into a comparatively tranquil sea in which the silt was being gradually deposited, but there are only three known agencies by which this could have been affected, viz., volcanoes, floating trees, or floating ice.
The rock in some respects resembles a volcanic breccia, and the resemblance is occasionally far more marked than appears in the Simla rock, but the steadiness with which the rock preserves its character over large areas, the frequent occurrence of well-rounded pebbles, the comparative absence of actually angular fragments, and more especially the absence of associated lavas or volcanic ashes, tell against this hypothesis. Floating timber is known to transport fragments of rock often of considerable size, but it is difficult to imagine how the enormous number of stones found in the Blaini boulder slate could have been thus transported, nor why the boulders should be confined to certain well-defined horizons; it would be strange too if out of the vast number of trees that would be required to transport all the blocks found in the Blaini group, all escaped fossilization, and the absence of carbonaceous matter from the boulder beds is very marked when they are compared with the associated slates.
The only hypothesis that remains is that of floating ice, which presents no difficul
ties: floating ice is as capable of transporting waterworn pebbles as angular fragments without any limit of size; it melts away and leaves no trace beyond the fragments it may have carried with it, and, though the exact causes are not satisfactorily determined, it is a known fact that icebergs have formerly floated where none are now to be seen, and that regions now covered with perpetual snow and ice were once free from both. There is, consequently, no difficulty in accounting for the restriction of the transported blocks to certain definite horizons. Besides being the only hypothesis which does not land us in a difficulty there is a certain amount of corroborative evidence, for in 1884 Mr. C. S. Middlemiss discovered, in the Blaini group of the Naira (Neweli) valley in Eastern Sirmur, a pebble which was scratched in a manner very similar to that which is due to the action of glaciers, and I have myself seen in the neighbourbood of Simla several fragments which shew the same in a more or less obscure manner: in particular I may mention one block situated on the spur between Chadwick's falls and the glen which measures 4' x 2' x 1' exposed; on one face it is smoothed and scored in a manner that can hardly but be due to glacial action. Taking everything into consideration we may, therefore, decide that when the Blaini group was being deposited, the spot now occupied by Simla was a sea on whose surface icebergs floated, melted, and dropped the stones, which they carried, on their surface or imbedded in their substance.
Returning now to the description of the rocks, the Blaini group, as defined on the accompanying map, consists at the base of a boulder slate; above this come carbonaceous slates which differ from those above and below by the manner in which they turn white on the weathered surfaces. The same feature is occasionally seen in the other carbonaceous beds, but in this particular band it is, so far as I know, universal. Above this again comes the original Blaini group of Mr. Medlicott. At the base it consists of a dark red slate which passes upwards into grey, occasionally blackish and carbonaceous, slate through which boulders are scattered, and above that there comes a thin-bedded pink limestone. It is this group of beds, not more than 60 feet thick in all, that is most easily traced (many of the exposures having already been recorded by Colonel McMahon1) and has been followed more completely by me than any other boundary. It enters the map in a side ravine flowing into the main stream below the Chadwick falls and can be traced over the spur into the Chadwick ravine; up the east side of this ravine it cannot be traced continuously, being hidden by surface debris; it may be cut by a small fault here, but this must be of later small throw, for the beds reappear in their proper position on the crest of the spur; they can be traced down into the glen, being hidden near the top and again near the bottom by soil cap, but, for the greater part of the way, can be traced practically continuously and closely follow one of the native footpaths. In the bed of the stream flowing through the glen the limestone is seen and is cut by a small fault which brings it into immediate contact with the red slates. It is again seen on the ascent from the glen and can be traced over the spur separating the glen from Annandale and runs down to where the road round this spur passes the spring which supplies the village of Gowai (of the map) with water. Here the limestone disappears, being apparently cut off by a fault; it cannot be traced across the Annandale valley owing to the rocks
'Records, G. S. I., X.
being hidden by sub-recent stream deposits, but may be seen in the stream bed and on the road which leads up the spur. East of Annandale it suddenly reappears, being evidently brought up by a fault. As the road rounds the end of the spur to turn back on itself, the red slates appear overlaid by the boulder bed and the limestone, here only represented by a few weathered blocks, the greater portion having apparently been removed by solution. The limestone and conglomerate can be traced with fair continuity over this spur along its western side, where again a native footpath follows it very closely into the ravines below the ridge. I did not follow it here, but it reappears below the ridge and can be traced up, passing below the Lakar Bazaar, on to the road round the north side of Jacko, where it crosses a spur and appears to run down into the valley west of Snowdon, but the ground is so covered that I was not able to trace it. Going along the North Jacko road the Blaini comes in once more, rather suddenly, as if brought up by a small fault, and runs past Snowdon, where there is a very characteristic exposure of the conglomerate just outside the guard-house. The limestone appears to run down under shady dell, but where not built on the ground is so covered with surface debris or spoil that has been tipped in levelling ground for roads or buildings, that it is impossible to follow the beds. However this may be, the Blaini is not again seen till just beyond the ravine where the upper and lower roads round Jacko separate. The beds are here brought up by a fault which shows itself as a line of springs. From here the Blaini can be traced across the upper road and up the spur to below the summer house in the grounds of Holly oak; here it appears to be again cut by a fault and is next seen just above where the road turns round the end of Jacko; there is only a small patch here, the beds cannot be traced in either direction, nor do they shew on either of the roads along the south side of Jacko. The limestone and conglomerate may however be found in the ravine below the Simla Rifle Club's range, west of and about level with the village of Sangati (Sanguti). I did not see it again till just above Sháman (Chaman) village where it can be traced practically continuously past Balahi, beyond which the limestone has been quarried. It runs down the west side of the Chota Chelsea spur and is seen near the Dhobi ghât below Chota Simla: hence it can be traced at intervals along the east slopes of the continuation of the Chota Simla spur till it runs out of the limits of the map.
It is noteworthy that the limestone is always well seen on the hill sides facing southwards, while on the northern face, which is covered with forest, the outcrop is always thin and often wanting. It is a known fact that the air contained in the interstices of the soil of a forest contains a larger proportion of carbonic acid gas than that of open ground; the water which percolates this soil would consequently become more impregnated with the gas and would be a more active solvent of the limestone than that which percolated the soil of the southern slopes. To this fact seems attributable the thinness or absence of outcrops of limestone on the northern slopes.
The lower conglomerate I did not trace with the same continuity as the upper one. It is seen in the Chadwick stream and again on the spur separating that from the glen. I next saw it where the road from the glen to Annandale passes over the spur, and it can be traced round the spur east of Annandale. On the Elysium spur