THE ORIGIN OF THE INCLUSIONS IN DIKES 5 



nounced solvent action on the inclusions, either the basic limestone or the more 

 acid granites; neither has the heat greatly affected the limestone, which one 

 would expect to find in a crystalline state. 



There are a number of other breccia dikes in this immediate 

 vicinity, 2 to 7 feet wide, of both a camptonite and a pyroxenite 

 composition. They contain fragments of granite, gneiss, essexite, 

 syenite, Potsdam sandstone, and Trenton Hmestone, the country 

 rock. All these inclusions have either come from below or from the 

 limestone near by. 



It is evident that the inclusions in the cases above mentioned 

 have been derived almost wholly by the dike-magma shattering 

 the walls as it advanced. There must have been an active circu- 

 lation in the dikes to keep them at a high temperature at the top 

 while they made their way through the limestone and probably 

 through the underlying rocks. This circulation carried up the 

 blocks of the pre-Cambrian and carried down the blocks of the Pots- 

 dam and Beekmantown. These dikes appear, therefore, to be cases 

 where the ascent was due, at least partly, to a blowpiping action on 

 the rock overhead. 



In the s^me memoir Harvie describes other dikes and intrusions 

 of various forms, all of which consist of a breccia of similar inclu- 

 sions in a basic matrix. One of the masses of breccia on St. Helen's 

 Island,"" near Montreal, cutting Utica shale, contains blocks of 

 fossihferous Oriskany limestone and Helderberg limestone, indicat- 

 ing the former presence of strata of these ages in the region. It 

 also contains fragments of the Ordovician and older rocks. 



The vertical movement of the fragments in these cases is inter- 

 esting because it has been in both directions in the same dike or 

 mass. This movement does not seem to be dependent on the 

 form of the intrusion nor on the composition of the intruding dike- 

 rock. The thickness of the formations from the top of the Lauren- 

 tian to the bottom of the Utica is 2,500 feet (Harvie). The Utica 

 is succeeded by Lorraine shales 2,000 feet thick. No higher strati- 

 graphic horizon is exposed in the region, except in the inclusions 

 on St. Helen's Island. Therefore these Devonian fragments must 



^ See also F. D. Adams, Twelfth Internal. Geol. Cong., Guide Book 3, p. 55, for a 

 description of these breccias. 



