64 STRUCTURAL AND FIELD GEOLOGY 
opposite the mouths of many of our mountain valleys. These have been 
transported by the glaciers of the Ice Age as superficial moraines, some 
of the more conspicuous heaps having been dumped down as terminal 
moraines during long pauses in the final retreat of the old ice-flows, while’ 
hummocky sheets of the same materials were gradually spread over the 
flanks and bottoms of our mountain valleys as the glee melted more or 
less rapidly away. : 
II. Chemically-formed Rocks 
These are for the most part chemical precipitates from 
aqueous solutions, and are chiefly calcareous, siliceous, ferru- 
ginous, and saline. Some have been deposited at the surface 
as the result of evaporation; others are precipitates from 
saturated solutions, and have thus accumulated on the floors 
of salt-lakes and seas. Again, a few are of the nature of 
aggregations: originally diffused through the rocks in which 
they occur, they have since drawn together and become con- 
centrated so as to form nodules and nodular masses or 
independent layers. 
Stalactites and Stalagmites.—These are precipitates from 
water holding calcium carbonate in solution. They are of 
common occurrence in limestone caverns, the stalactites 
growing downwards from the roof and the stalagmites 
gradually accreting on the floor. Carbonated water percolat- 
ing through the limestone oozes out on the roof of a cavern, 
and, being there exposed to evaporation, is compelled to part 
with some of its calcium carbonate, which adheres to the 
rock-surface. When the gathering drop of water falls to the 
ground it is there again exposed to evaporation, and gives up 
the remainder of the carbonate which it held in solution. 
The colour of these deposits varies indefinitely—they may be 
creamy-white, yellowish, brownish, or reddish, and are often 
mottled. They usually show a concentric, laminated structure, 
and the stalactites, in the early period of growth, are porous 
and readily crushed; subsequently, however, their pores 
become filled up with calcium carbonate, and the structure 
thus gradually solidifies. Stalagmites are seldom or never 
so porous, but exhibit a well-defined laminated structure (see 
Plate XIX.). In course of time both stalactites and stalag- 
mites, owing to molecular changes, tend to acquire a crystalline 
structure. 
