8 SIDNEY POWERS 



conglomerate from which the pebbles were derived (Hedstrom's 

 theory). The inclusions of quartzite are generally surrounded 

 by zones in which iron ore, biotite, and chlorite are developed. The 

 contact of the diabase with the quartzite pebbles is, however, 

 usually sharp, while the contact with the sandstone and granite is 

 more or less indistinct, the magma having formed fused contacts 

 and even having penetrated a few of the inclusions. In places a 

 flow structure is developed in the diabase around the fragments. 



Eichstadt proposes the theory that the inclusions in both dikes 

 were derived from loose pebbles on the surface of the ground, prob- 

 ably accumulated in valleys, which fell into fissures formed in 

 advance of the diabasic intrusions and were caught in the magma 

 when it ascended in the fissures. • In support of this theory he points 

 out that if the blocks of quartzite were originally angular and the 

 edges resorbed where they are now, the silica content of the diabase 

 near the zones of inclusions would be much greater than elsewhere 

 in the dike. This is not the case, although he finds free quartz and 

 micropegmatite present in the diabase of both dikes. He noted the 

 existence of pre-Cambrian conglomerate in the vicinity, but con- 

 siders that the pebbles were derived from a loosely consolidated 

 conglomerate (as this Almesakra conglomerate may have been in the 

 pre-Cambrian time when the dikes were formed) and not from a 

 massive conglomerate. 



Hedstrom proposes a somewhat similar theory : that larger and 

 smaller pieces of the Almesakra conglomerate have been imbedded 

 in the diabase magma. He considers that the conglomerate was at 

 least somewhat consoHdated and accounts for part of the matrix 

 by the numerous grains of quartz and feldspar scattered in the 

 diabase. In support of this view he finds inclusions to which por- 

 tions of the original matrix are still attached. The hard quartzite 

 pebbles in the Ahnesakra conglomerate are greatly cracked and 

 show the same cataclastic structure as do those of the inclusions 

 which have often split up into several pieces which have been more 

 or less widely displaced from each other in the magma. The 

 cement of the original conglomerate was gritty, loose, and brittle, 

 and therefore its cohesion was easily destroyed at the intrusion of 

 the diabase. "In the appearance and character of the pebbles, as 



