INTRODUCTION. 



additions have had to be made in our original lists of formations. 

 For instance, the ' Tonbo Series ' of Mr. Datta, which both of us 

 placed at the base of the fossiliferous sequence, has since been 

 shown to be identical, at least in part, with the limestone of 

 the plateau, and to be younger than the Silurian. Again, one 

 very important formation of Silurian age, the upper Namhsim marls, 

 which is touched at only one point by the railway, and at another 

 on the cart-road, received very little attention ; while the lower part 

 of the same formation, the Namhsim Sandstone, escaped notice 

 altogether. 



Modifications of our preliminary investigations were made in 

 subsequent General Reports of the Geological Survey, but as they 

 are now superseded by the present Memoir there is no need to 

 refer to them in detail here. Mr. Datta was associated with me 

 in the survey until the year 1903, and I have been assisted at 

 different times by my colleagues Dr. G. E. Pilgrim and Mr. Coggin 

 Brown, references to whose work will be found in the following 

 pages. 



Even now the present Memoir must be considered, not as a 

 complete account of the geology of the Northern 



General considerations. m a, 1 1 .. 



bhan states, but merely as a preliminary 

 attempt to bring into order the observations made by my col- 

 leagues and myself ; to furnish a basis, which I can only trust may 

 be found to possess a certain degree of stability, on which future 

 geologists may work ; and to connect, as far as possible, the 

 geology of this interesting tract with that of the surrounding 

 regions. The area dealt with in these pages is almost twice the 

 size of that of Wales, and it contains a sequence of rocks that 

 for variety and complexity of structure may be compared with 

 those that are found in that country, but with very few facilities 

 for observation. It is covered for the most part with forest, 

 or with grass so tall and dense that it is often impossible to force 

 one's way through it ; traversed by few roads, and those con- 

 structed at the least possible cost, and only driven through rock 

 when it is absolutely necessary, while the same may be said of 

 the only railway ; while the mule tracks, the usual means of 

 communication, are little better than footpaths. There are no 

 quarries, which are of so great assistance in countries where 

 stone is used for building and road-metalling, for every house 

 is built of timber or bamboo mats ; and there are very few 



