40 WALKS AND TALKS. 



Syenitic Gneiss contains very little feldspar, we call the rock 

 Hornblende Schist. 



The names above explained embrace a majority of the 

 rocks, and I do noj; by any means expect you to learn at 

 present all of the others. But we may mention a few. Some 

 rocks appear to be composed wholly of one mineral, and 

 yet they are not quartzites. They are mostly dark-col- 

 ored slate-colored or blackish or greenish. If these are 

 banded in different colors, or are capable of splitting into 

 sheets, like shingles, they are Argillites including most of the 

 hard slates, like roofing slate. If a rock is very fine, black- 

 ish, and harder than slate, it may be an Aphanite. If it con- 

 sists of a very fine, hard, uniform reddish or greenish base, 

 having crystals of feldspar scattered through it, the rock is a 

 Porphyry. But, if you feel inclined to go further into rock- 

 details, it will be better to study some work which takes up 

 the subject more thoroughly. (See the Author's Geological 

 Excursions, and still fuller Geological Studies). 



One word more. These bowlder rocks are all hard, crys- 

 talline, and generally (not always) foreign to the region where 

 they lie. We sometimes find fragments of rocks which are 

 not hard and crystalline, and far-fetched. They come from 

 ledges which appear at the surface not far away. The most 

 common uncrystalline ledges are of sandstone, limestone, and 

 shale. Sandstone is composed chiefly of grains of quartz like 

 those in a granular quartzite, but not so brilliant or so firmly 

 compacted together. A grindstone is a fine sandstone. Lime- 

 stone can be easily scratched with a knife ; is generally not com- 

 posed of grains ; and, if you apply strong acid, an effervescence 

 takes place. Very strong vinegar will often suffice, especially 

 if the rock is first pulverized. Shale is quite soft, easily 

 cut with a knife, dark-bluish or black, splitting into thin 

 sheets. It is exceedingly common in coal regions. It crum- 

 bles into small fragments, and finally becomes mere mud. 



I think this little knowledge about rocks is better than 

 total ignorance. We may now go amongst our bowlders, and 

 speak many of their names. 



