FROZEN SEAS OP LAVA. 95 



therefore a shallow- water deposit, while the columnar basalt is 

 three thousand five hundred feet thick. High cliffs of basaltic 

 columns, like those exposed on the Hudson and Columbia 

 rivers are often called palisades. 



\ In some cases the uprising lavas have not been able to 

 find their way to the surface. Either the fissures in which 

 they started, from some unknown depth, never extended to 

 the surface, or the streams lost their way and found them- 

 selves pent in the strata, and crowded in every direction in 

 search of relief. In such case the lavas have sometimes in- 

 sinuated themselves laterally between the strata to such ex- 

 tent as to separate the strata by a considerable interval, with- 

 out being able to escape to the surface. The result is, a dome- 

 like elevation of the surface, forming a peculiar type of mount- 

 ains called laccolites named and first described by Mr. G. K. 

 Gilbert. In many cases, the arched strata become much fis- 

 sured, and the lava sheets communicate quite freely with each 

 other. Such laccolites exposed to the processes of erosion reveal 

 the constitution of the interior. This subject is fully illus- 

 trated by Mr. Gilbert in his memoir on the Henry Mountains 

 in eastern Utah. 



River erosions of vast lava-sheets have resulted in many 

 striking forms. As the most extensive sheets are the result 

 of late geological action, they generally rest on incoherent 

 materials gravel and sand, as in Oregon and California. 

 When the erosions of the streams have cut through the lava, 

 and for some distance into the gravel, the less coherent nature 

 of the latter causes an undermining of the lava-sheet. It thus 

 projects like a table-top, beyond the gravel. When the ero- 

 sion cuts the lava-sheet along parallel lines, it gives rise to 

 the forms known as "table mountains." These are common 

 in the volcanic region of central France; and especially so in 

 eastern California. In Butte county, the ancient drainage 

 wore channels stretching westward from the upland of the 

 Sierra. These were subsequently filled by outflows of lava. 

 Then in modern times, a change of levels established drainage 

 from north to south. The modern streams, therefore, have 



