136 GEOLOGY. 



angles, as these illustrations would in- 

 dicate, but curves, and therefore anti- 

 clinal and synclinal curves are spoken 

 of in the anticlinal the sides inclin- 

 ing upward toward each other like the 

 roof of a house, but in a curved line, 

 while the sides of a synclinal curve in- 

 cline downward toward each other, 

 like the sides of a trough. 



220. Measuring Strata. Where strata outcrop we can 

 very readily take the measurement of their thickness. 

 The manner in which this is done is so well pointed out 

 by Mr. Jukes, Director of the Geological Survey of Ire- 

 land, that I will give you his description of it entire : 

 " Having procured a good map of the district we are go- 

 ing to examine, a pocket compass, and a small instrument 

 called a clinometer, by which we can determine the an- 

 gle at which a bed inclines from the horizontal plane, we 

 begin to look for exposures of rock. The district may 

 perhaps at first appear to be entirely covered by soil and 

 vegetation, but, when thoroughly examined, it may show 

 here and there some crags of bare rock, some bare cliffs 

 on the side of a river, some cutting on a road-side, or 

 some quarries. Let Fig. 64 be a piece of our map trav- 

 ersed by a brook and a road, and suppose that we find 

 some quarries of limestone in the northeast corner of the 

 district, all the beds of which dip to the southwest at an 

 angle of 20. Let us represent these limestones by the 

 cross-barred lines in the northeast corner of the figure, 

 drawing them in a northwest and southeast direction to 

 represent their strike, and indicating their dip by an ar- 

 row with 20 annexed to it. In examining the banks of 

 the brook, suppose we find, in two places lying in the 

 same line of strike, some shales, represented by plain 

 close lines ; a bed of coal, represented by a thick dark 

 line; and some sandstones, represented by dots; and, 

 for simplicity's sake, let us suppose them all to dip south- 



