36G ME. H. B. MILXER ON THE [vol. Ixxvili^ 



Topaz. Invariably clear, colourless and transparent, and in irre- 



gular flakes. 



Andalusite. Well individualized as regards optical properties, parti- 

 cularly that of pleochroism, which is intense in many 

 grains. Often well crystallized, although usually turbid 

 with alteration-products such as kaolin ; contrasting, 

 strongly with topaz in these respects. 



The foregoing species have been selected on account of their 

 importance quantitatively, and on account of their value for- 

 purposes of correlation. 



The marked similarit}' of the mineralogical composition of the 

 sediments and of the more precise physical and optical characters, 

 of the component species, can only be explained on the assumption 

 of derivation from a homogeneous ' distributive province,' using 

 the phrase to include both igneous and sedimentary facies.^ Quite 

 apart from general lithological resemblance and similar mode of 

 occurrence of the deposits, we find that (excepting two doubtful 

 minerals), in onl}'^ four cases are species non-persistent throughout,, 

 and of these all save one (kj^anite) are comparativeh^ unimportant. 

 Such uniformit}^ in composition is most significant, and is parti- 

 cularly striking in the present case. Again, the frequency of 

 occurrence of the important species, and the similarity of 

 crystallographical and optical properties, and of the nature and 

 degree of abrasion exhibited by these species, are factors which 

 we cannot overlook, and are strongly suggestive of the truth of 

 the hypothesis above postulated. It would seem not unreason- 

 able, then, to conclude that the deposits were the product of a 

 definite period of erosion, subsequent denudation, probably marine,, 

 determining the removal of much of the detritus which had accu- 

 mulated ; the survival of a few isolated remnants of the original 

 mass was by no means fortuitous, but due entirel}^ to favourable 

 conditions of deposition. On this basis, the initial assumption of 

 contemporaneity of the deposits under discussion becomes a distinct 

 probability, and receives support not only from the mineralogical 

 evidence cited above, but also from topographical criteria to be 

 discussed in the sequel. 



YII. SorscE OF Oeigijst or the Deposits. 



Any Avell-differentiated phase of detrital deposition must of 

 necessity give rise to sediments reflecting the nature and composi- 

 tion of the parent rocks of the comminuted material ; it follows, 

 therefore, that an enquiry into the normal petrological habitat of' 

 the several miineral species concerned should furnish us with, 

 valuable evidence as to the source of origin of the deposits, which, 

 when combined with the geological and phj^siographical knowledge 

 obtainable from the area to-daj', should serve as a foundation for 

 a geographical reconstruction of this part of Britain at the epoch 

 under consideration. The results of such an enquiry show that 



'^ The term ' distributive province ' was suggested by Mr. A. Brammall, M.Sc». 



