420 E. S. MOORE 
well developed at almost every place where the surface of the 
basalt is exposed (Fig. 7). Outside of these two bodies all the 
igneous rock seen showed definite evidence of intrusive origin. 
There does not appear to be any definite relation between the 
origin of the iron-formation and these igneous rocks, because it 
does not seem to matter whether they intrude it or are flows over- 
lying it, whether they are close to it or are far removed from it. 
There is in many places a little micaceous and specular hematite 
near the contact between the diabase and the adjacent sediments, 
but this seems to be independent of the original iron-formation. 
Fic. 5.—Large diabase dike cutting the jaspilite near Kasegalick Lake. It 
apparently served as a feeder for the large flow shown in Fig. 4, since the flow lies 
over the jaspilite just beyond the upper left-hand corner of the picture. 
THE ALGAL CONCRETIONARY LIMESTONES 
In recent years much attention has been paid to the study of the 
minute organisms which play an important réle as precipitating 
agents in calcareous and iron-bearing solutions. It has been proved 
that low forms of piants, chiefly the algae, are at the present day 
causing to be precipitated great quantities of calcium carbonate 
in streams, lakes, and seas, and that the iron bacteria are respon- 
sible for the deposition of much iron in bogs and other bodies of 
water. Dr. Walcott set a new record when he described the 
numerous algal structures from the pre-Cambrian rocks of Mon- 
