342 PRE-CAMBRIAX VOLCANIC BOMBS 



chocolate drops and inering-ues. Pieces of the fractured crust 

 of some of the smaller balls appear to have fallen out and been 

 lost at the time. 



I have j^et to speak of another feature, and that not 

 of any lesser interest than the foregoing. Parallel to the 

 major diameter of several of the balls, there is a more or 

 less complete striation or grooving around the circumference. 

 The grooves may be single or double, and have a width of as 

 much as an eighth of an inch, while the striaj are more numer- 

 ous and in a belt of fourteen or more on a scale of even less 

 than thirty to the inch. In the specimen on tlie table this 

 equatorial linear engraving can best be seen when light is made 

 to fall parallel to the axis of rotation. It is further to be seen 

 passing under the remains of an adhesion giving priority to 

 its formation ; but how this engraving was produced, or how 

 the tool which made the lines was held, I am at a loss to suggest. 



We may now consider how came these balls of matter, 

 doubtless volcanic ashes, to acquire their present form, and 

 under what conditions would it seem most probable they 

 were produced ? Whatever may be the ultimate concensus 

 of opinion, it seems to me their formation can best be con- 

 ceived by comparison with that of a modern volcanic bomb, 

 and is due to svvirling gases of an explosion giving a 

 gyratory motion to the ejected particles of attrition and their 

 acraregations. 



Messrs. Chamberlain and Sali§bury, in their recent work on 

 geology,* write : — " The larger masses of lava ejected into the air 

 are often caused to rotate by the unequal force of the projection, 

 or by the unequal friction of the air, and to assume spheroidal 

 forms. . . . These rounded projectiles are known as volcanic 

 bombs." Of the ejected dust from Vesuvius they further say — 

 " A finer variety [than lapilli] of the nature of sand, much used 

 in making Portland cement, is locally known as puzzolana." 

 This latter quotation is made as bearing on the rapid cementa- 



*T. C. Chamberlain and K. D. Salisbury. Geology, 1904, vol. 1, p. 386. 



