344 PRE- CAMBRIAN VOLCANIC BOMBS 



jectiona belonging to characteristic bombs. We have therefore 

 to assume some marked change in the conditions to meet their 

 peculiarities, for the special interest which these balls of 

 undoubtedly igneous origin possess, lies not merely in their being 

 volcanic bombs of very ancient geological horizon but in the 

 presumption that they indicate phenomena of unusual type and 

 a rapid cementation of volcanic ashes of a composition differing 

 from those of ordinary modern volcanoes which, generally highly 

 basic, make a mud which of itself does not possess the property 

 of speedily setting as a cement. 



The value of intimate grinding to an extreme fineness is 

 well known in the manufacture of commerical cement, but 

 conditions could not be reproduced which are effected during a 

 volcanic explosion when the separation of impalpable dust is 

 under the influence of the chemical agency of supei'heated steam 

 and other gases, and which for the moment, it is hero assumed, 

 existed when these bombs were taking their natal flight. The 

 late exhibition of vulcanism at Martinique and St. Vincent 

 makes the contemplation of a rapid setting cem nt within 

 reason, but requires that the composition of the powder be 

 otherwise than that met with in the ejecta of the Lesser Antilles. 



A complete comparison is not at present possible, as no 

 ultimate analysis of the Lake Ainslie ash-bed has been made. 

 That it materially differs from that of the dust from the Wind- 

 ward Islands is evident. Analysis showed the latter to be very 

 basic, with an avei-age of 55% of silica at St. Vincent, and 62^ 

 of silica at Martinique ; while from the accompanying letter 

 from Dr. Hoffmann of the Geological Survey it is clear the silica 

 contents of the Lake Ainslie rock is very much higher. 



Ottawa, August 8th, 1904. 

 My dear Mr. Poole. 



I duly received 3'our letter of the 30th ult., as likewise the 

 specimens therein referred to. With regard to the " concretions 

 of felsite." In so far as composition is concerned, they co. sist 

 essentially of silica, with a little alumina, etc., etc. The amount 

 of alumina, in the specimen examined, was comparatively small 



