[HARViE] PALAEOZOIC BRECCIA OF THE VICINITY OF MONTREAL 277 

 graphically much higher tlian the Utica, namely the Helderberg and 

 Onskany. The distance which these fragments would have to descend 

 m order to reach their present position, is rather uncertain, but may be 

 roughly stated. Sir William Logan estimated the thickness of the forma- 

 tions from the bottom of the Utica to the top of the Laurentian at 2,050 

 feet. Eecent borings for wells in the city of Montreal show that these 

 figures should be increased by at least 500 feet. The Utica is succeeded 

 by the Lorraine shales with a thickness in this vicinity of 2,000 feet. 

 The formations later than the Lorraine are not found in this neighbour- 

 hood, with the exception of these few inclusions. It is therefore quite 

 impossible to say exactly, what movement the inclusions represent, but it 

 must be much greater than the 2,000 feet represented by the Lon-aine. 

 An important fact revealed by the petrographical study is that in 

 every case the cement of the breccia was in a molten condition both be- 

 fore and after the inclusion of the fragments. The well crystallised forms 

 of the components of the matrix would alone leave no doubt on this point, 

 the fact is, however, further emphasised by a study of the contacts. In 

 every case where the exposures admit of an examination of the contact, 

 it is found that the breccia acts as an intrusive. The igneous material 

 must have been in a molten condition and frequently impelled by a force 

 sufficient to shatter the wall rock driving the fiuid magma into the cre- 

 vices thus formed. This feature is illustrated in a most striking manner 

 by the breccia dyke in the rear wall of Little's quarry described on page 

 — . At La Trappe, the Grenville limestone near the contact is shot 

 through by tongues of the igneous paste of the breccia. On lie Bizard 

 large blocks of the surrounding limestone have been upheaved by the 

 breccia. Another instance is the intrusive contact between the breccia 

 and the shale on Ste. Helen's Island, described on page — . These facts 

 render untenable the hypothesis put forward by Miss I^olan and Miss 

 Dixon in which the breccia is supposed to be derived from a volcanic ash. 

 The peculiar position of the blocks which are the sole representatives 

 of the Helderberg and Oriskany in this district, has been the source of 

 frequent speculation. Since it has now been shown that the paste was in 

 a molten condition when it enclosed the fragments, and that the breccia 

 as a whole has acted as an intrusive, the explanation is rendered com- 

 paratively simple. The breccia represents the truncated pipe or outlet 

 of a reservoir of molten igneous material, which outlet may have reached 

 the surface and even formed a subsidiary cone to Mount Eoyal, or else it 

 may have been of the nature of a laccolitic mass, not opening on the sur- 

 face. In either case the intrusion extended up into the Helderberg and 

 Oriskany which must have overlain the Utica. The action of the intru- 

 sive stoped off blocks from these higher strata which then either sank of 



