Reviews — The Origin of the Hiinalcbyan Folding. 167 



Finally, T would point out that the main object of my article is 

 likely to be lost sight of in a controversy about the precise line of 

 division between two zones and the inclusion of certain beds and 

 certain fossils in the one or the other or both. I wish to concentrate 

 attention on the main question, and on this we are happily all agreed, 

 that it is desirable to divide the old zone of A. quadratus into two 

 distinct zones, a lower and an upper. Future research must determine 

 whether the two zones in the south are approximately coterminous 

 with the two similar zones in the north or not, and whether the 

 difference between the two faunas is as great in England as it seems 

 to be in Germany. 



I. — The Origin of the Himalayan Folding. 



A MEMOIR recently published in India by Colonel S. G. Burrard, 

 C.S.I., F.R.S.,^ suggests an explanation of the Himalayan folds 

 which geologists cannot alford to pass in silence, as his geodetic work 

 points to conclusions that are at variance with the generally expressed 

 view regarding the immediate cause of overthrusting in such folded 

 ranges. The commonly accepted mental picture of the Himalayan 

 folds depicts the strata as pushed over by great tangential thrusts 

 from the north, the movement being resisted by the stable and ancient 

 Horst forming the Indian Peninsula. 



Colonel Burrard's results suggest that this view of the mechanism 

 of Himalayan folding is inconsistent with the fact that, parallel to 

 the general fold axis, along the southern foot of the range, and 

 extending for about 15 miles outside the visible foot of the hills, 

 there is a band which is characterized by an extraordinary deficiency 

 in gravity. The observations so far made show that this band is 

 especially sharp and abruptly marked on its northern margin, that is 

 along the foot of the range, and that the deficiency in gravity 

 gradually diminishes towards the south. The gravity values are 

 those that might be expected if we had an empty fissure of about 

 6 miles deep, or a partiallj^ filled cavity of very mucli greater depth. 



Colonel Burrard insists that the deficiency is altogether too great 

 to be due to a simple synclinal depression filled with light alluvium, 

 and he points out that, whatever be its nature, one cannot conceive 

 of a band of low gravity being caused by the thrusting of a mass of 

 rock against the peninsular Horst. He is thus forced to adopt 

 a complete reversal of the accepted mechanical system of overthrust- 

 folding, and to contemplate the existence of a deep-seated fissure 

 opening out by a northerly creep of the sub-crust, while the overlying 

 sedimentary carpet becomes puckered and wrinkled to accommodate 

 itself to its shortened base. These wrinkles form the Himalayan 

 ranges, the southerly-directed overthrust-faults and folds being due, 

 not to a positive pushing over from the north of the superficial film 

 of rock, but to a pulling back of the deep-seated support. 



^ On the Origin of the Himalaija Mountains (Professional Paper No. 12, 

 1912, Survey of India, Calcutta). 



