Fossils. 



What they are. — Very frequently we notice in fragments 

 of rocks certain marks or imprints that seem to have been made 

 by some organic being. They sometimes recall the shape and 

 markings of certain sea-shells of today. As we break open more 

 rocks and study more carefully all the markings, moulds and im- 

 pressions we find the conclusion is irresistible that they were 

 made by some kind of by-gone sea-shells, often quite different 

 from those we see on the beaches to-day. 



How they came to be in the Devonian rocks of Southern New 

 York. —Sea-shells in solid rock? How came they there? These 

 are the queries that perplexed even the wisest minds up to within 

 a comparatively recent date; queries, too, that the collector must 

 be ready to explain to every man who espies him and comes 

 to see what he is collecting. 



* f<titn! 







20 



>\ 



Observation. — Outcrops of one and the same layer may often be identi- 

 fied at considerable distances from each other. To determine the direction 



and amount of dip when three 

 r ^ too >». suoh outcrops are known, see 



; " the solution (graphic) of the 



following problem: A rock 

 Jcrtr layer is found on one side of 



a valley at B, 200 metres 

 higher than it is at A, 2 kilo- 

 metres distant. At C, on the 

 opposite side of the valley, its 

 altitude above A, 2.1 kilo- 

 metres distant, is 100 metres. 

 The distance between C and 

 Bis 1.65 kilometre. The di- 

 rection of the line C — B is 

 north 20 west. Required 

 the direction and amount of 

 w "" dip of this layer. 



Fig- 2* Find by proportion the 



point on A — B at an equal altitude with C. It is at TJ in this instance, half 

 way from A to B, Draw C — TJ and continue it a short distance. Drop A — V 

 perpendicular to C — TJ extended, and continue it to W. Measure angle A 

 W B (here about 74 ); this is the direction of dip, i. e. ? north about 74 

 west. V is by construction on C — V the line of strike 100 metres above A. 

 Thhn if the distance A — V is found by measurement to be about .96 kil.=96o 

 metres then 100—960 is the amount of dip sought. 







F 



is* 

 * kScm, 





B % all, 200 m 



