168 BASA'LTIC HIiLOCKS, OR BOSSES. 



vided, and terminating on a plain horizontal surface. When the 

 mass is composed of several stories, the separations are sometimes 

 formed by thin beds of rapilli, and most generally they are dis- 

 tinguished by alternations of compact and porous matter, which 

 characterizes each particular effusion. 



5. These characters leave no doubt as to the igneous origin of 

 these deposits ; but there are still others. When we can penetrate 



beneath basa'ltic tables, as in cases 

 where they rest on moveable forma- 

 tions, we almost always find the in- 

 ferior part of the mass presents a 

 multitude of appendages (Jig. 276), 

 which penetrate into the soil, indi- 

 cating a liquid matter that has been 

 moulded in rents or crevices. The 

 earth on which the mass rests is 

 often found calcined through a great- 

 er or less thickness, and the debris 

 Fig. 276. Appendages of basa'lt of plants it contains are carbonised. 

 in subjacent rocks. On the other hand, there is often 



found on the surface of basa'ltic tables points of scorification, par- 

 ticular elevations, and even crate'riform depressions, towards which 

 the melted matter seems to have retired at a certain moment before 

 solidifying. 



6. Basa'ltic hillocks, or bosses, are of different kinds ; some 

 seem to be the remnants of an extensive table which had been 

 partly destroyed ; in this case the principal mass of the bosse be- 

 longs to one or another species of soil, and the summit only is 

 basa'ltic. In others, on the contrary, the whole hillock is formed 

 of basa'lt, and the base is lost in masses of sand and debris, which 

 prevent us from seeing what is beneath ; some others are attached 

 to veins or seams. The composition of these hillocks, like that of 

 tabular basalt, varies. 



7. jRasa'ltic veins, or seams. Basa'lt is frequently found in 

 reins. Most frequently the mass of the seam or vein is compact, 

 or irregularly cracked, "but it is often divided into prisms, perpen- 

 dicular to the parietes of the crevice, which then become the 

 cooling surfaces (fig* 277). The matters in these seams are rarely 

 scorified, but some instances are met in Vivarais and Auvergne. 

 Most frequently basa'ltic veins are prolonged to the surface of the 

 soil, where they present their out-crop ; but it frequently happens, 

 also, they terminate above in pointed masses (Jig. 278), sometimes 

 bifurcated, which are lost in the rocks through which they pass. 



4. What are the characters of tabular basa'lt ? 



5. What is the origin of basa'lt ? 



6. What are the characters of bosses of basa'lt 7 

 7 What are the characters of basa'ltic veins ? 



