236 IRVING. 



But not only are sheets and laccolites characteristic types of 

 intrusions in the Cambrian formation, but dikes and irregular 

 bodies are very extensively developed in the regions of greatest 

 eruptive activity. There is, however, in the most irregular 

 masses often a readily traceable connection between the form 

 which the eruptive has assumed and the character of the rock 

 into which it has been intruded. An excellent illustration of 

 this can be seen in the railroad cut at Portland. A cut has 

 been made through the upper measures of the Cambrian, reveal- 

 ing a few feet of heavy sandstone, over which lie some 1 5 feet of 

 Cambrian shales. These are of an extremely fine, fissile, char- 

 acter, separating easily into the very thinnest of plates. A cross 

 jointing has further been developed, so that they break in all 

 directions with equal facility. Into these have been intruded 

 masses of quartz-porphyry which present a veiy peculiar ellip- 

 tical form, as if the railroad had cut across the arm-like exten- 

 sion of an irregular eruptive mass. When examined closely, 

 however, they prove to be merely nuclei of comparatively unde- 

 composed porphyry, which are connected with very irregular 

 branching masses. These run out in all directions, but from 

 their decomposed condition and the partial covering of shales, 

 which have fallen from above, they are not to be observed at 

 first sight. Plate XVI is a photograph of one of these masses 

 in which the line between the irregular porphyry and the shales 

 has not been completely obscured. 



Not only is there a marked contrast between the intrusions 

 in the Cambrian and the Algonkian, but an even greater one 

 manifests itself as we pass from these formations up into the 

 limestones of the Carboniferous. 



Instead of the innumerable intrusive masses that dot the 

 Cambrian areas, the integrity of the limestones is disturbed by 

 very few, for the massive character of the rock, and its great 

 thickness have been an effectual barrier to the upward passage 

 of the igneous rock. Dikes, as, for instance, the biotite phono- 

 lite dike in Spearfish cafion, on reaching the Carboniferous 

 have been unable to penetrate it and have become "blind." 

 Where vertical fissures have allowed the igneous rock to pene- 



