278 GEOLOGY OF THE SECOND DISTRICT. 



From tlie preceding remarks, it will be understood tiiat this rock has less thickness in this 

 county than usual, but still it is to be regarded as a mass very well characterized. In pur- 

 suing the geology, however, of this valley, this rock is an important member in this series, 

 and it is sufficiently well brought out to preserve the order and relations of the masses in this 

 particular field. 



Utica Slate. 



This rock being only partially exposed, and occupying only some of the most depressed 

 portions of the county, cannot be so fully described as some of the lower rocks of the Cham- 

 plain group. It appears first near Split-rock, occupying the southern side of Whallon's bay. 

 At this place it lies against or upon the primary, in consequence of its extension in a particu- 

 lar direction ; or it appears here, as along some other extended lines, to overlap the rocks 

 beneath, and to rest visibly upon the primary ; that portion only which extends beyond the 

 masses beneath it, being in contact with the oldest or Primary system. 



This mass at Split-rock is worthy of examination, in consequence of the curvatures in the 

 strata at the line of junction ; from which it appears, that since the consolidation of the slate, 

 this portion of the Primary sj^stem has been elevated at least in part. 



Proceeding along the shore, we find the remains of a porphyry dyke in imperfect columns 

 in the slate. No change in the texture or structure of the slate appears. The Triarthus, 

 and numerous specimens of Graptolites which arc found within one or two inches of the 

 porphyry, appear as in the same mass when remote from a plutonic rock. Neither are the 

 layers disturbed or contorted, in the vicinity of this injected rock. 



We may trace the utica slate on the shore as far as the uplift of the cliazy limestone, a 

 distance of a mile. After leaving this mass, which contains the porphyry, we soon find it 

 disturbed. It now dips on this shore in all directions, and its layers are more or less bent. 

 When it comes in contact with the chazy limestone, as has been described, it is injected with 

 numerous trap dykes, and traversed by veins of calcareous spar. 



After this interruption by the disturbances at this place, the utica slate does not appear until 

 about half a mile north of Essex. It there occupies the whole shore as far as Peru bay. It 

 forms only low banks, and here is barely exposed, so that we see merely the upper surface. 

 Two miles north, it is more exposed near the road than along the shore, by a small stream 

 which has cut a clianncl through it fcr seventy or eighty rods. At this place, it is again tra- 

 versed by several dykes, one of which is a porphyry resembling that near Split-rock, and at 

 Canon's point. 



After searching a long lime for fossils in this mass, I was unable to find any ; though from 

 its being disturbed, the unsuccessful search does not prove their absence. 



This rock is the highest in the series in this county, and it is confined to Essex and the 

 lake shore for a few miles north. In it we find no important mineral substances, and inas- 

 much as it is britllc and sul)ject to decomposition, it is never suitable for roofing. After 

 drying, and then licing moistened, it is liable to crack and fall to pieces ; and by a want of 

 cohesion, it is generally channelled out by small streams which flow over it. 



