
- into limonite, the interior remaining 
Parr IT. Sect. ii. § 1.] WEATHERING. 335 
and the remaining soluble parts are found as a crumbling crust. 
Impure limestone, however, yields a weathered crust of more or less 
insoluble particles. Hence, as we have already seen, the relative 
purity of limestones may be roughly determined by comparing their 
weathered surfaces, where, if they contain much sand, the grains will 
be seen projecting from the calcareous matrix ; should the rock be 
very ferruginous, the yellow hydrous peroxide or ochre will be found 
as a powdery crust, or if the rock be fossiliferous, the weathered 
surface will commonly present the fossils standing out in relief. An 
experienced fossil collector will always search well these weathered 
limestones, for he often finds there, delicately picked out by 
the weather, minute and frail fossils which are wholly invisible 
on a freshly broken surface of the stone. This difference arises 
from the greater insolubility of the crystalline calcite composing 
the organic remains than of the more granular calcite in which they 
are imbedded. 
Rocks liable to little chemical change are best fitted to resist 
weathering, provided their particles have sufficient cohesion to with- 
stand the mechanical processes of disintegration. Siliceous sandstones 
offer excellent examples of this permanence. Consisting mainly of 
the durable mineral quartz, they are sometimes able so to withstand 
decay that buildings made of them still retain, after the lapse of 
centuries, the chisel-marks of the builders. Many sandstones, how- 
ever, contain argillaceous, calcareous, or ferruginous concretions 
which weather more rapidly than the rock, and cause it to assume a 
honeycombed surface ; others are full of a diffused cement (clay, lime, 
iron), the decay of which causes the rock to crumble down into 
sand. In sandstones, as indeed in most stratified rocks, there is a 
tendency towards more rapid weathering along the planes of strati- 
fication, so that the stratified structure is 
brought out very clearly on natural cliffs 
(Fig. 84). In many ferruginous sand- 
stones and clay ironstones successive 
yellow or brown zones or shells may be 
traced inward from the surface, frequently 
due to changes of the ferrous carbonate 

still fresh. In many prismatic massive Fie. 85.—Rinas or WEATHERING. 
rocks (basalt, diorite, &c.) segments of 
the prisms weather into spheroids, in which successive weathered 
rings form crusts like the concentric coats of an onion (Kigs. 85, 86). 
Where one of these rocks has been intruded as a dyke, it sometimes 
decomposes to a considerable depth into a mass of brown ferruginous 
balls im a surrounding sandy matrix—the whole having at first a 
resemblance to a conglomerate made of rolled and transported 
fragments (Fig. 87). 
No rock presents greater variety of weathering than granite. 
Some remarkably durable kinds only yield slowly at the edges of 
