BASA'LTIC HILLOCKS, OR BOSSES. 



169 



Fig. 277. View of prismatic 

 basa'lt. 



Fig. 278. Basaltic seams of 

 Villeneuve-de-Berg. 



This circumstance positively indicates that the basa'lt was not in- 

 troduced from above, and that it could only have been injected 

 from the interior towards the exterior of the earth. Sometimes 

 the vein glides betwixt two strata, which it follows to a greater or 

 less extent ; or, in ramifying, it launches a part of its mass into 

 the interval, and ends by terminating there in a corner, whence it 

 spreads into ati the little fissures of the rock. 



8. Along the course of basaltic 

 veins, the out-crops of which are 

 seen on the surface of the soil, 

 various isolated hillocks are fre- 

 quently observed (fig. 279), seve- 

 ral together at various distances 

 apart, which appear to be nothing 

 more than partial ejections, like 

 the cones formed along the same 

 crack in modern volcanic erup- 

 tions. Most often they are almost 



entirely composed of scoriae, but Fig. 279. Hillocks on the course of 



some are found which consist of a vein. 



pure basa'lt. Sometimes, instead of hillocks, there are effusions 



of tables of more or less thickness (fig. 280), which are also 



found along the course of a 



vein. All these circumstances 



lend to explain the formation 



of isolated hillocks, as well as 



the series of hillocks in line, Fi ^ 280.-F.in terminating in a table. 



found in a great many localities where the internal vein has found 

 here and there an outlet. 



9. fiction of basa'lt on adjacent rocks. The calcination of 

 clays, and the carbonisation of vegetable debris lying beneath ba- 

 sa'lt, have been mentioned ; granite traversed ty veins of it is very 

 much altered, portions of rocks which have been enveloped ; n 



a How are isolated hillocks of basa'lt accounted for ? 

 15 



