TERTIARY VOLCAxNIC ROCKS 261 



may never reach the surface except by side fissures. Yet the order of eruption 

 would be from acid to basic wherever the several lavas have flowed throuerh the 

 same vent, and where flows from difterent vents have not intermingled or over- 

 lapped." 



It is of course necessary to read carefully the whole of Dr. Jensen's valuable 

 report in order to appreciate the arguments which have led him to the above im- 

 portant conclusion. This is published in Vol. II. of this Memoir. 



2 L 



