970 A. L. BARROWS FOSSIL ROCK-BORING ANIMALS . 



at the surf line undoubtedly lias been a factor in instigating the boring 

 habit in the cases of the boring sea-urchins and mytilids. It seems prob- 

 able that these types of borers will usually be found at no very great depth 

 below the surface of the sea, — that is, above 10 or 15 fathoms. These 

 types as fossils may therefore be regarded as fairly close determiners of 

 the approximate location of ancient coastlines. 



Pholad borers, however, which are of a different derivation, may be 

 found considerably deeper. Boulders of serpentine and of partially de- 

 composed gabbro containing pholad borings and living pholads have been 

 dredged in the Golden Gate, at the entrance to San Francisco Bay, from 

 depths ranging between 33 and 50 fathoms. The geology of the Golden 

 Gate makes it seem probable that these boulders were entered by the 

 borers at or near the places where they were picked up. On the whole, it 

 seems probable that even pholad borers will be found most abundantly 

 under usual coastal conditions at depths ranging from the surface to only 

 25 or 30 fathoms, except in special places where the rocks are scoured 

 bare by strong currents. Thus these fossils may also become indicators 

 of the proximity of a former shoreline, but without the definiteness of the 

 my til id borers in this respect. 



Evidence furnished by Rock-borers 



In the 23resence of a violent cliange in the conditions of deposition over 

 a given area, involving the laying down of hitherto absent sediments, the 

 free-living fauna may readily move away. The sessile fauna may also be 

 destroyed and washed away, and may leave but few remnants on the spot 

 on which it once flourished; but the boring forms imprisoned within 

 the rocky substratum must there remain, and may thus constitute the 

 only relics of the former fauna indicative of conditions at this place at 

 tlie time of inundation by the immediately overlying sediments. As a 

 matter of field experience, fossil rock-borers are usually found at a con- 

 tact between an old, more or less eroded surface and a bed of more recent 

 material, where remains of the superficial fauna of the old surface rarely 

 appear. The practical value of rock-borers as usually the only relics of 

 the former fauna to be secured at the given horizon is, therefore, in- 

 creased. 



In two other respects rock-borers speak with definiteness : First, in 

 differentiating absolutely between a suspected fault and a disconformity, 

 and, secondly, in sometimes pointing witli clearness to important discon- 

 formities which might otherwise be obscure. They may also be of use in 

 confirming the exact location of a non-confoi-mity between beds of dif- 



