MUSEUM OF COMPARATIVE ZOOLOGY. 19 



the layers of conglomerate a thickness of from twenty to thirty feet 

 may often be found in which only slight evidences of bedding are ascer- 

 tainable. 



This section has been subjected to two classes of accidents. One has 

 resulted in the general tilting of the deposits in an easterly direction, 

 the angle of inclination not usually exceeding about 20° of slope. At 

 certain points in the field there have been disruptions of the rocks, with 

 possible faulting on the lines of breakage, attended by the extrusion of 

 massive dikes of hornblendic granite, which appear in the form of elon- 

 gated somewhat lenticular-shaped ridges, which taper abruptly at either 

 end. These ejections vary in diameter from a few hundred feet to as 

 much as a mile, and their greatest length in one or two cases may 

 amount to three miles or more. On either side of the injected syenite 

 (or hornblendic granite) the bed rocks are folded abruptly upward into 

 vertical attitudes, which sometimes continue for a considerable distance 

 from the face of the dike material. 



It is an interesting feature connected with these intrusions of gran- 

 itic matter, that in no case do they appear to have brought about any 

 very conspicuous metamorphism in the sedimentary deposits with which 

 they have come in contact. The change is rarely apparent at more than 

 two or three feet from the dike. Actual contact has been seen but at 

 one point, southeast of locality No. 3 on the map, where the slates are 

 found in almost immediate juxtaposition with the hornblendic granites. 

 At locality No. 1 we have an extensive area exposed within one hun- 

 dred feet of the contact with the hornblendic granite mass, more than 

 half a mile in diameter. At this point we find no perceptible meta- 

 morphic influence on the sedimentary strata. 



The topographic features within the limits of the Cambrian field are 

 in a large measure determined by the resistance to erosion aff'orded by 

 these elongate domes of ejected matter. In a less determined way the 

 ridges of conglomerate influence the shape of the country. The horn- 

 blendic granites are but rarely exposed to the eye, for the reason that, 

 wearing evenly, they form a uniform surface on which the drift material 

 rests as a blanket. The conglomerate ridges, wearing irregularly, often 

 appear as sharp peaks too steep to retain any considerable coating of 

 glacial detritus. 



It is probable that these granitic ejections took place before the depo- 

 sition of the Carboniferous rocks, for, although a very marked feature in 

 the Cambrian district, they have nowhere been observed penetrating 

 through or into the coal measures which overlie them. Indeed, as I 



