ON CONGLOMERATED ROCKS. 



2. Cotemporaneous veins of basalt and amyg«- 

 daloid traverse the tuff*. 



3. The tuff itself sometimes occurs in globu- 

 lar concretions, and these concretions are again 

 composed of curved lamellar concretions. 



4f, Some varieties of tuff, like basalt and green- 

 stone, are entirely composed of globular distinct 

 concretions. 



5. Cotemporaneous veins of tuff traverse it in 

 all directions. 



6. Cotemporaneous veins, partly of the nature 

 of tuff, partly of the nature of basalt, occur in 

 the tuff. 



7. The masses of basalt and other rocks in the 

 tuff, are sometimes of such an enormous size, and 

 so constructed of easily separable concretions, as 

 to banish every idea of their being rolled frag- 

 ments. 



8. Lastly, The quantityfof mechanical intermix- 

 ture, is in general inconsiderable, and it is prin- 

 cipally composed of fragments of primitive, tran- 

 sition, and older ficetz rocks. 



From the preceding descriptions and observa- 

 tions, it follows, that the quantity of mechanical 

 mineral matter in the crust of the earth, is far 

 less considerable than has been generally supposed. 



* These veins agree in so many characters with the veins of 

 basalt, porphyry, granite, pitchstone, &c. usually considered-of 

 after-formation, that I am inclined to view many^of these also as 

 of a cotemporaneous nature. 



