536 B. K. N. Wyllie & A. Scott— Plutonics of Garabal Hill. 



between the two stations, due to the effect of the less dense material 

 filling the Gangetic trough, could not be less than 20" and might 

 amount to over 30". From tliis it appears that without going beyond 

 the known facts of the geological structure of the region — facts which 

 are independent of any theory of the origin of mountains or the 

 constitution of the interior of the earth — we can account for nearly, 

 if not quite, the whole of the unexplained residual deflexions, which, 

 instead of amounting to 23" and 7", would not come to more than 

 3" or 4", and the difference of 30" would disappear oi", at the least, 

 be reduced to one of a few seconds of arc. 



In all that has gone before I have avoided the question of the 

 origin of mountain ranges in general, or of the Himalayas in 

 particular, as, although I entered on the investigation, which I hope 

 to publish in detail, with the hope that the limit of fairly established 

 deduction might be carried further into the domain of pure speculation, 

 I have found that the geodetic results do not give any material 

 assistance. They have confirmed some conjectural inferences 

 regarding the form of the rocky bed of the Gangetic depression, 

 but on the question of the origin of the Himalayas, and of the nature 

 of their support, the evidence is too uncertain and equivocal to be of 

 any material value. This much, however, seems certain, that there 

 is no good evidence for the existence of a rift of 20 miles or so in 

 depth, as, once the known facts of geological structure are taken into 

 consideration, the existence of such a rift would explain too much and 

 introduce fresh difficulties even greater than those for which it was 

 introduced as an explanation. Moreover, we must add to this 

 negative evidence the positive fact that every observer, in every 

 part of the range which has been visited, has found evidence of 

 compression in precisely that zone wliere Colonel Burrard's postulate 

 demands extension. 



III. — The Plutonic E,ocks of Gaeabal Hill. 



By B. K. N. Wyllie, M.A., B.Sc, and Alexander Scott, M.A., B.Sc, 

 Carnegie Kesearch Scholars in the University of Glasgow. 



{Concluded from the November Number, p. 508.) 



AT various places, such as the north side of Garabal Hill, the two 

 Garabal burns (loc. iv and v, Fig. 1), and elsewhere, remarkably 

 coarse rocks are found. A good section showing an apparent passage 

 from tonalite to a coarse hornblenilite is exposed at loc. v (Fig. 1). 

 The i"ock in the bed of the burn is the normal tonalite, which appears 

 to pass gradually to diorite. A closer examination of the unweathered 

 rocks, however, shows that the passage is only apparent, and that 

 the tonalite is clearly intrusive into the diorite with sharp junctions. 

 The diorite near the junction is a rock of porphyritic aspect, con- 

 taining large crystals of zoned diopside, abundant green hornblende 

 and felspar, while the amount of quartz is small. A good deal of 

 biotite is present, so that the rock might be described as a pyroxene- 

 mica-diorite. Within a few feet the rock becomes coarser in texture, 

 and the felspathic content diminishes, while there is a corresponding 



