BIBLIOGRAPHY. 189 
Subjoint. Minor joints diverging from or parallel to the regular joints. (See 
P. 41.) 
Syncline. A geological term for the trough part of a wavelike sheet or bed of 
rock. 
Till. A mixture of clay and bowlders deposited by glaciers. 
Toeing-in. Quarrymen's term for the wedging in of the end of a granite sheet 
under an overhanging joint, probably in consequence of the faulting of the 
sheets along the joint. It is also applied to the overlapping of lenticular 
sheets. 
Twin crystals. Two adjacent crystals which have formed with the poles of 
their main axes in opposite or different directions. (See p. 20.) 
Weathering. The decomposition of a rock owing to the action of the weather. 
(Seep. 54.) 
