. The building of an island : being a sketch of the geological structure of the Danish West Indian island of St. Croix, or Santa Cruz. Geology -- Virgin Islands of the United States Saint Croix. 'IIIE ; OK AN ISLAND. 19 As \vr shall find later, this question of dip takes a very important place in the studv of the history of the rock formations, and is, therefore, worth special attention. Occasionally the layers are horizontal and then there can he no question of dip; hut commonly this is not the case, the rocks generally slope in some definite direction, that is tf) say, towards some
. The building of an island : being a sketch of the geological structure of the Danish West Indian island of St. Croix, or Santa Cruz. Geology -- Virgin Islands of the United States Saint Croix. 'IIIE ; OK AN ISLAND. 19 As \vr shall find later, this question of dip takes a very important place in the studv of the history of the rock formations, and is, therefore, worth special attention. Occasionally the layers are horizontal and then there can he no question of dip; hut commonly this is not the case, the rocks generally slope in some definite direction, that is tf) say, towards some definite point of the compass, and this is noted down as the dip; the layers mentioned ahove, for example, have a souihwest dip. The amount of the dip is measured in angular degrees from the horizontal, and after a little practice we can generally tell by the eye alone the approximate amount of the dip; hut if we wish to measure it more accuratelv, we must use a simple instrument called a clinometer (slope- measurer). Such an instrument can be bought from an instrument maker;, but may also be constructed at home on a card with sutlicient accuracy for amateur purposes. The figure below shows such a home-made clinometer. The Card A B has a semicircle drawn on it, on the edge of which the degrees from one to ninetv are marked off on both sides in an upward direc- tion, starting at D. From the centre C a small shot is suspended by a thread, and the lower edge of the card is applied to the layer of rock. The shot, of course, hangs perpendicularlv antl shows the angle at which the layer under examination slopes from the horizontal. As the beds of rock often present a rather rough surface, it will be found convenient first to rest a straight stick along the rock in the direction of the dip and then to apply the clinometer to the stick, instead of to the rock Please note that these images are extracted from scanned page images that may have been digitally enhanced for readabil
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Keywords: ., bookcentury1900, bookcollectionbiod, bookdecade1900, bookyear1907
