242 IRVING. 



able sheets of a rock, which is chemically and mineralos^ically 

 almost precisely similar, are found lying between the horizontal 

 Cambrian shales not more than a mile distant. In the opposite 

 direction in Squaw creek, at even less distance, dikes and sheets 

 of phonolite occur in great numbers. Again the rock of Crow 

 peak may be duplicated in many sheets and dikes not far dis- 

 tant, as may also that which forms the Needles. Can we then 

 imagine two rock masses intruded at the same time, under the 

 same conditions, and of the same chemical composition, to be 

 of a highly fluid character in one place, and sufficiently viscid 

 to form a " plutonic plug " in another at no appreciable distance ? 



But if we are not to explain the form of these larger intru- 

 sions by the assumption of a high degree of viscidity, we most 

 look elsewhere for the causes that have determined it, and the 

 explanation is to be found in the character of the formations 

 into which the magmas have been intruded, and the local 

 violence of the force which has intruded them. 



Attention has already been called to the contrast between the 

 form of intrusions characteristic of the three separate forma- 

 tions — Algonkian, Cambrian and Carboniferous — to the pre- 

 dominance of dikes in the slate areas, of sheets and laccolites 

 in the Cambrain, and to the comparative, lack of intrusions in 

 the limestone formation. The last named are veiy limited in 

 character, or else intruded in a formation below, and exposed 

 above the limestone area by the erosion of the uplifted cover- 

 ing. The lines of least resistance in the first instance have been 

 vertical, and the only type of intrusion has been dikes ; in the 

 second they have been horizontal and sheets have resulted, and 

 when finally the Carboniferous has been reached, the massive 

 limestone has been so resistant a formation that it has prevented 

 the further passage of the igneous rock. Where the force 

 of intrusion has been more violent, however, and the mass of 

 intruded material great, there has not been the same opportu- 

 nity for lateral expansion and the mass has domed up the more 

 resisting beds, sometimes only slightly, forming a gently sloping 

 laccolite, sometimes to a much greater degree, so that the elas- 

 ticity of the overlying rock, which would naturally be less than 



