40 



EXTERNAL STRUCTURE OF ROCKS. 



arranged over each other in regular layers ; the particles or frag- 

 ments of which they are composed vary in size, and indicate 

 the different states of agitation or repose of the waters from which 

 they were deposited. It is proper to notice, that certain rocks 

 are disposed to divide in parallel seams, in a different direction from 

 that of the regular stratification : this results from the crystalline struc- 

 ture of the rock. Some strata appear to have been formed by chem- 

 ical precipitation ; and, not unfrequently, chemical precipitation and 

 sedimentary deposition have taken place, at the same time, and pro- 

 duced rocks of a mixed character. 



The Tabular structure consists of parallel plates of rock, separa- 

 ted by regular seams. This structure has, often, been confounded 

 with stratification : it appears to be the result of crystallization, and 

 is closely allied to the columnar structure. 



The Columnar ox Prismatic structure is peculiar to certain rocks, but 

 occurs chiefly in the basahic and volcanic class. Thick beds are 

 divided into columns or prisms, which are, most generally, pentago- 

 nal. They, sometimes, form vast ranges of natural columns, as at 

 Staffa, the Giants' Causeway in Ireland, and in many volcanic coun- 

 tries. Sometimes, the prismatic structure may be observed forming 

 detached groups of columns and prisms, as represented in the group 

 of columns on Cader Idris. (Plate VII.) A group of basaltic col- 

 umns of similar form, and equally perfect, was observed by the au- 

 thor on the side of the volcanic mountain called Gravenaire, in Au- 

 vergne, at a small distance from the crater. 



The Globular structure consists of globular masses, either detach- 

 ed or imbedded in rocks of the same kind; they are frequently com- 

 posed of concentric layers. 



The terms Massive^ or Indeterminate, may be applied to all un- 

 stratified rocks that have no regular divisions. Many of the prima- 

 ry rocks, such as granite, porphyry and serpentine, occur in masses 

 of enormous thickness, which are broken by irregular fissures in ev- 

 ery direction. Thick currents of lava, which have filled up hollows 

 or valleys, are also indeterminate, as might be expected from their 

 mode of formation. Sometimes rocks of granite and porphyry, and 

 also of compact lava, present either a tabular or columnar structure; 

 but the structure is seldom so regular as in basaltic rocks. 



