MAPPING OF THE CRYSTALLINE SCHISTS 791 



6. Variable hardness of rock. — Ridges are occupied by harder 

 rock, valleys by softer or more soluble rock. To be applied with 

 great caution. In many districts, in the opinion of the writer, 

 the valleys are nearly if not quite as often hard as soft rock. 

 A structural line of weakness may have produced a pre-glacial 

 stream bed which the ice-cap widened and deepened; an ante- 

 cedent stream may have produced the same result in hard 

 rocks, an orographic block may have suffered depression, etc. 



7. Hydrography. — In a limestone and hard rock country lakes, 

 ponds, rivers, and swamps are generally i?i softer or more soluble 

 rock. A most pernicious doctrine because so generally accepted 

 and so frequently wrong. Faith in the canon also discourages 

 investigation. The writer has in mind a nearly circular lake in 

 the middle of a great valley largely underlaid by limestone, but 

 about one-half the lake bottom is mica schist, the other half 

 limestone. 



8. Ridge and valley structure. — Ridges are more generally syn- 

 clines and valleys anticlines. A half truth. It is perhaps true in 

 the greater number of instances of rocks which suffer an amount 

 of plication and rupturing of flexures (schists, slates, and some- 

 times limestones). It is not true of the heavy gneisses which 

 so generally raise their heads in domes, the beds quite generally 

 dipping with the slopes. 



9. Blocks. — Angular blocks when not lying on steep slopes 

 and not clearly ice borne are on bed rock of the same kind. The 

 tendency is to drive this doctrine to its limit. For great lime- 

 stone blocks not strengthened by silica or silicates it may per- 

 haps be assumed that they are not very far from the parent ledge. 

 With a framework of silica they have been carried by the ice 

 thirty miles or more. 



10. Sink-hole structure. — Superficially undrained areas when not 

 morainal [or in drift) are o?i limestone. A pretty safe rule. 



1 1 . Soil. — In a limestone and hard rock coimtry arable farm land 

 is on limestone and woodland on hard rock. A fairly useful rule if 

 applied intelligently. 



Canons of mapping modified by basal assumptions. — The 

 above canons are to be considered in connection with one 



