The semicolon comes third in the descending order of punctuation: period, colon, semicolon, comma. It indicates a pause or degree of separation less than a colon but more than a comma.

The semicolon is used in these instances:

1. to separate clauses that are too closely related in meaning to be written separately:
   
   
   Phyllitic slate, phyllite, and fine-grained, argillaceous sandstone are the dominant rock types; they are penetratively cleaved and weather greenish grey and brown.
   
   
   
2. to separate closely related clauses in a compound sentence where the connecting conjunction is omitted:
   
   
   In this section there are many graptolitic beds; in section 2, there are none.
   
   
   
3. to separate principal clauses in a long sentence from phrases or subordinate clauses marked off by commas:
   
   
   The succession is composed of the Carnival Formation, a quartz sandstone unit; the Greentree Formation, a marine shale; and the Blackbird Formation, a second, less extensive sandstone unit.
   
   
   
4. between the clauses of a compound sentence when there is a contrast of ideas:
   
   
   In GSC publications, we refer to a fine-grained sandstone, not a fine sandstone; a coarse-grained granite, not a coarse granite.
   
   
   
5. before a conjunctive adverb (therefore, however, moreover, indeed, in fact, that is, for example, consequently, and furthermore) when it connects independent clauses:
   
   
   Corrections can be made for variations in topography along a survey line during data processing; however, surface conditions and the depth to the water table are likely to vary with the topography, and these changes may affect the frequency characteristics and resolution of the data.
   
   A semicolon is not used before a conjunctive adverb if it is being used in a transitional sense (i.e. not joining independent clauses):
   
   
   Uranium-lead dating, however, suggests that the volcanic rocks in this succession are not significantly younger than the Coldbrook Group (Bevier et al., 1994).
   Therefore, although great uncertainties remain, the most sophisticated models available for developing long-term predictions of climate change suggest that the Palliser Triangle region will be warmer, and likely drier, in the future.

Semicolons following quotations and parentheses should be placed outside the closing quotation mark and parenthesis.