FAULT NEAR BOULDER. 117 



The normal fault (PI. XIII, fig. 1 ) is a fracture of which the plane A A 

 is inclined to the planes of stratification B B, and along which the hanging 

 wall C lias glided upon the foot wall 1> with reference to a given stratum in 

 such a manner as to form the downthrown side: it must furthermore be 

 made a part of the definition that under those conditions any line // — no 

 part Of which lies within either of the acute angles formed by the planes of 

 the fault and stratification in such a manner as to make with either plane a 

 second acute angle of smaller value than the former, and wholly contained 

 within it — will fail to cut a uiven thickness or series of beds equal in 

 amount to the stratigraphical throw of the fault; on the contrary, a line g g 

 lying within either of the acute angles x x, as above described, would, if 

 produced, twice cut a series of beds equal to the stratigraphical throw. 

 The normal fault may also he designated as one of extension, from the 

 stretching apart laterally of the beds. 



The vertical fault (PI. XIII, fig. 2) is one of which the plane is perpen- 

 dicular to the stratification planes, either side being downthrown without 

 effect upon the definition. The beds are neither stretched nor compressed 

 laterally. 



In the reverse fault ( PI. XIII, fig. 3) the plane A A is also inclined to 

 the planes of stratification B B — but not of necessity to the plane of the 

 horizon or to a vertical lin< — and along the fault plane the hanging wall ( 1 

 has glided upon the foot wall D, with reference to a given stratum, in such 

 a manner as to form the upthrown side; it must furthermore be made a 

 part of the definition that under these conditions any litn — no part of 

 which lies within either of the acute angles formed by the planes of the 

 fault and stratification in such a manner as to make with either plane a 

 second acute angle of smaller value than the former, and wholly contained 

 within it — will twice cut a given thickness or series of beds equal in amount 

 to the stratigraphical throw of the fault: on the contrary, a line // g lying 

 within either of the acute angles, as above described, would fail, if produced, 

 to cut a series of beds equal to the stratigraphical throw. The "reverse" 

 fault may also lie designated as one of compression. 



Professor de Margerie and Ileim' and 0. Fisher- have defined the three 



'Les dislocations de I'e'corce terrestre, p. 22. < Seologica] Mugaziue, 1884, pp. 204 et seq. 



