Geological and topographical maps, their interpretation and use, a handbook for the geologist and civil engineer . Fig. 68.—Section ofLaccolite. cannot be readily distinguished by the forms of theiroutcrops. Sills or Intrusive Sheets.—The typical intrusive sillis of great lateral extent in comparison to its thickness,which is usually fairly constant, thus distinguishing it fromthe laccolite. When sills occur in sedimentary strata they usuallyfollow the bedding planes, and thus resemble lava sills are, however, usually wider in extent and moreregular as regards their thickness than ar
Geological and topographical maps, their interpretation and use, a handbook for the geologist and civil engineer . Fig. 68.—Section ofLaccolite. cannot be readily distinguished by the forms of theiroutcrops. Sills or Intrusive Sheets.—The typical intrusive sillis of great lateral extent in comparison to its thickness,which is usually fairly constant, thus distinguishing it fromthe laccolite. When sills occur in sedimentary strata they usuallyfollow the bedding planes, and thus resemble lava sills are, however, usually wider in extent and moreregular as regards their thickness than are lava streams. In some instances sills branch and in others change. Fig. 69,—Section of an Intrusive Sheet or Sill, showingChange of Horizon and Branching. their geological horizon one or more times, as indicated inFig. 69, and either of these phenomena, when present,serves to distinguish them from lava flows. In the field it is possible to distinguish the sill by thefact that it metamorphoses both the rocks on which it 86 GEOLOGICAL AND TOPOGRAPHICAL M\P3 rests and also those above it, whereas the lava flow can, ofcourse, only produce changes in the floor over which itflows, but unless the metarnorphic rocks are indicatedupon the map this means of identification is closedto us. The above phenomena are well shown by the GreatWhin Sill of the North of England, which is intrusive inthe Carboniferous rocks of the Pennine Chain, and whichchanges its horizon by a number of steps amountingin all to nearly 1000 feet, while on Thistle Green andWiddybank Fell, saccharoid limestone, which is a productof contact metamorphism, is shown in contact with theupper
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