142 THE CRYSTAL FALLS IRON-BEARING DISTRICT. 



the fragments in close contact, and also to fill the intervening spaces with 

 fine detritus. Those cases must of course be excepted where the material 

 fell upon water so deep that after sinking to the bottom the action of the 

 Avaves was not felt. Under such circumstances one can imagine the blocks, 

 being partly supported b}^ the w^ater, coming to rest in a more unstable 

 position than they would in the air. 



The cement differs in different specimens. The minerals constituting 

 it are quartz, feldspar, calcite, chlorite, epidote, and hornblende. The 

 minerals are found including one another in such a way as to make it prob- 

 able that they usually formed simultaneously. The calcite is an exception, 

 as it is usually present in greater quantity near the surface of the exposures, 

 and is therefore a weathering product. It was noted above that the horn- 

 blende and chlorite frequently extend from the rock fragments into the 

 clearer elements composing the cement. Hornblende needles in many 

 cases constitute a large part of the cement. Where two fragments are very 

 close together, a perfect network of needles may extend from the one across 

 the intervening space and penetrate the other, and the fragments thus prac- 

 tically grow together. The cavities — especially the large ones mentioned 

 above — have quite frequently been filled with concentric growths of vari- 

 ous minerals. In general, chlorite seems to be the first mineral deposited, 

 and quartz the last, but in weathered specimens calcite is the last. 



FINE TUFFS OK ASH (DUST) BEDS. 



The fine tuffs or "ash" beds occur plentifully in the Crj'-stal Falls district. 

 In many cases they possess a very well developed cleavage, and were very 

 puzzling in the field on account of their striking resemblance to true sedi- 

 mentary slates. They are of interest as emphasizing the resemblance 

 between pi-e-Cambriau ejectamenta and Tertiary and Recent ones. In one 

 respect they differ from the modern forms. The dust from Krakatao in 

 1885 and from other volcanic explosions consists mainly of fragments of 

 minerals and glass. The constituents of the Crystal Falls beds are usually 

 fine lava and glass fragments and less commonly minerals. 



The rock fragments are angular, vesicular, and completely altered. 

 The glass fragments are likewise angular, and have the characteristic curved 

 shapes from which they are usually described as sickle-shaped bodies. (Fig. 

 B, PI. XXXII.) Such are formed when a pumice is broken up, and each 



