BASA'LTIC DEPOSITS OF fUFFErtENT KINDS 167 



surface, we may conclude they were accumulated on a horizontal 

 soil, or that in a more or less liquid state they flowed into a depres- 

 sion. They are evidently deposits which have issued from the 

 bosorn of the earth in a state of fusion. It is by observations of 

 this kind we are enabled to recognise extinct volcanoes, in relation 

 to which the history of the most remote times is entirely mute. 



2. Some of these currents resemble what is called basa'lt, that 

 is, black rocks with a compact base of la'bradorite, containing 

 black pyroxene, and almost always magnetic oxide of iron. Very 

 frequently there is found in it more or less voluminous nodules of 

 peiidote, and sometimes crystals of feldspar, which give it a por- 

 phyritic structure. These currents ordinarily form thick deposits, 

 frequently divided into prismatic columns, sometimes in large 

 irregular pieces, all indicative of slow cooling. "The palisades" on 

 the North River are examples of basa'ltic columns. 



3. Basa'ltic deposits of different kinds. If basa'lt is found 

 in well-ascertained currents, traceable to craters, entirely similar 

 matter is found in very different positions. There is a great deal 

 of it that forms extensive tables of considerable thickness, consti- 

 tuting vast plateaux ; or heaped-up fragments on different moun- 

 tains, at the same level, the heaps corresponding, and seem to be- 

 long one to the other like parts of the same whole, showing a 

 vast dislocated table. Basa'lt also forms isolated masses, hillocks 

 in the midst of places, sometimes very distant from every 

 other formation of the same kind. It is found in seams, sometimes 

 enclosed in the soil that conceals it, sometimes rising here and 

 there like a wall, or presenting various hillocks on the same line 

 of direction. 



All these dispositions of basa'ltic deposits, as well as currents or streams, 

 are sometimes found together in the same country. In some countries, on 

 the contrary, there is no trace whatever of volcanic cones or of currents. 

 In all cases, however, the rock possesses the general characters of basa'lt, 

 and seems to rest indifferently on every kind of formation, even on vegeta 

 ble earth. 



4. Tabular basa'lt brings to 

 mind the great tables of Iceland, 

 especially those of the eruption 

 of 1783; they possess all the 

 characters of lava that has been 

 arrested on horizontal planes, or 

 filled depressions. The lower part 

 is compact, crystalline, and most 

 frequently divided into vertical 

 prismatic columns (Jig- 275) ; 

 and the upper part is porous, eel- Fig . 



lular, sco'riform, irregularly di- porous basa'lt. 



9. What is basa'lt ? What does it contain ? What is its form 2 

 3. Where is basa'lt found, and under what circumstances ? 

 35* 



