THE ORIGIN OF THE INCLUSIONS IN DIKES 7 



included in the dike, The contact of the pebbles and the diabase 

 in the case of this dike is sometimes sharp and sometimes indistinct. 

 In the pebbles, which have undergone the most marked absorption, 

 it is difficult to distinguish even the center of the pebble. Some- 

 times ilmenite and fine feldspar crystals have formed in the pebbles 

 with a rim of epidote and chlorite between the pebble and the dia- 

 base. The dike cuts Archean gneiss, and none of the younger 

 pre-Cambrian sedimentary series is present in the region. 



The "pebble-diabase" dike of Brevik (on the Eksjo sheet) has 

 a strike of about N.-S. and has been traced for about 15 miles. It 

 cuts Almesakra pre-Cambrian sandstone, quartzite, and con- 

 glomerate. The width of the dike varies in different exposures and 

 it often exceeds 300 feet. Inclusions appear only in some of the 

 outcrops, and there they are, as a rule, confined to smaller zones, 

 often 10 to 15 feet in width, which are elongated parallel to the sides 

 of the dike. These zones may be situated either on the sides of the 

 dike, and usually on the eastern side, or near the middle. They 

 vary in width from 4 to 50 feet, changing with the width of the dike. 

 In smaller offshoots from the dike, about 15 feet in width, fragments 

 sometimes occur evenly distributed over the entire width. Else- 

 where there is a sharp boundary between the parts free from and 

 full of inclusions. 



The inclusions consist largely of quartzite and schist derived 

 from the Almesakra pre-Cambrian complex, with some granites, 

 leptites, gneisses, and other pre-Cambrian rocks. The size of the 

 fragments varies from a few inches to 30 feet. The shape of the 

 quartzite inclusions is rounded, of the granite, subangular, while the 

 inclosures of schist form thin bands of considerable length, which 

 are surrounded by sheets of diabase. The pebbles are so numerous 

 as almost to touch. They occupy about half the volume of the 

 dike, and at times even more. The inclusions weather out easily. 



In the pebble-bearing parts of the diabase the ophitic diabasic 

 structure is not developed, and the presence of the numerous inclu- 

 sions has caused a segregation of the light and dark minerals into 

 separate spots. There are also grains of quartz and feldspar 

 scattered through the diabase as if derived from the resorption of 

 the edges of some of the inclusions or from the cement of the original 



