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178 GEOGNOSY. I 
discloses the fact, that while in some cases the weathered crust so 
completely obscures the essential character of a rock that its true 
nature might not be suspected, in other instances it is the weathered 
crust that best reveals the real structure of the mass. Spheroidal 
crusts of a decomposing yellow ferruginous earthy substance, for 
example, would hardly be identified as a compact dark basalt, yet, on — 
penetrating within these crusts, a central core of still undecomposed 
basalt may not unfrequently be discovered. Again, a block of lime- 
stone when broken open may present only a uniformly crystalline 
structure, yet if the weathered surface be examined, it will not 
improbably show many projecting fragments of shells, polyzoa, corals, 
[Book TL. 

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crinoids, or other organisms. The really fossiliferous nature of an ~ 
apparently unfossiliferous rock may thus be revealed by weathering. 
Many limestones also might from their fresh fracture be set down as 
tolerably pure carbonate of lime; but from the thick crust of yellow 
ochre on their weathered faces are seen to be highly ferruginous. 
Among crystalline rocks the weathered surface commonly throws 
light upon the mineral constitution of the mass, for some minerals 
decompose more rapidly than others, which are thus left isolated 
and more easily recognizable. In this manner the existence of — 
quartz in many felspathic rocks may be detected. Its minute blebs 
or crystals, which to the naked eye or lens are lost among the 
brilliant facettes of the felspars, stand out amid the dull clay into 
which these minerals are decomposed. 
The depth to which weathering extends should be noted. The 
student must not be too confident that he has reached its limit even 
when he comes to the solid more or less hard and splintery unde- 
composed stone. Granite sometimes decomposes into kaolin and sand 
to a depth of twenty or thirty feet. Limestones have often a mere 
film of crust, because their substance is almost entirely dissolved and 
removed by rain, 
With some practice the inspection of a weathered surface will 
frequently suffice to determine the true nature and name of a rock. 
Should this preliminary examination, and a comparison of weathered 
and unweathered surfaces, fail to afford the information sought, we 
proceed to apply some of the simple and useful tests available for field-— 
work. The lens will usually enable us to decide whether the rock 
is compact and apparently structureless, or crystalline, or fragmental. 
Having settled this point, we proceed to ascertain the hardness and 
colour of streak by scratching a fresh surface of the stone. A dro 
of weak acid placed upon the scratched surface or on the powder of : 
the streak may reveal the presence of carbonic acid. By practice 
considerable facility can be acquired in approximately estimating 
the specific gravity of rocks merely by the hand. The following 
tables may be of assistance, but it must be understood at the outset 
that a knowledge of rocks can never be gained from instructions 
given in books, but must be acquired by actual handling and study of 
the rovks themselves, : 
