GUELPH GROUP. 53 



or lower rocks, and it may therefore be considered a characteristic genus. Euca- 

 lyptocrinus has a wider geographical distribution, and is more abundant, and 

 for the same reason may be called characteristic. Orthis elegantula, 0. flabellum, 

 0. hybrida, Galymene blumenbachi, and llkenns barriensis have almost world-wide 

 distribution, and are characteristic of rocks of this age. 



$ 106. The dolomites of this Group in Canada are more or less bituminous. 

 In some parts of Western New York they contain so much solid bitumen that it 

 exudes from the rocks when heated. The escape of carbureted hydrogen from 

 these rocks is of common occurrence. Lyell described in 1841 a " burning 

 spring " on the river just above Niagara Falls, where the light hydro-carbon gas 

 rose from beneath the water out of the limestone rock. The invisible gas makes 

 its way in countless bubbles through the clear, transparent water, and on the ap- 

 plication of a lighted candle it plays about with a lambent, flickering flame, 

 which seldom touches the water, the gas being at first too pure to be inflammable^ 

 and only obtaining sufficient oxygen after mingling with the atmosphere at the 

 height of several inches above the surface of the river. This gas had its origin in 

 the shale, which forms the lower part of the Falls, and has found its way up 

 through 85 feet or more of quite compact limestone. Petroleum occurs in Niagara 

 limestone at Chicago, which had its origin in some shaly strata beneath, but arte- 

 sian boring failed to discover it in commercial quantities. Where gas or oil escapes 

 from surface limestone there is little prospect of finding accumulations of com- 

 mercial importance by artesian boring, because so much has escaped in the ages 

 which have passed away since the elevation of the limestone above the water of the 

 sea. There must be an impervious covering of clay or stone to retain such volatile 

 substances in valuable quantities. 



CHAPTER XV. 



GUELPH GROUP. 



§ 107. This Group was named from the town of Guelph in Canada, and de- 

 fined by Logan in 1863. It appears as a lenticular mass, resting upon the Niagara, 

 and having a maximum thickness of 160 feet. It is a limestone dolomite, particu- 

 larly distinguished for having no fossil Echinoderms, while it is rich in other 

 fossils closely allied to those in the Niagara, some of the species being identical. 

 It may have been a brackish water-deposit in an arm of the sea. It occurs in the 

 north-western part of Ohio with all the fossils and characteristics pertaining to it 

 in Canada, but is unknown elsewhere. It is doubtless of the same age as the Onon- 

 daga Group, and probably should not bear a distinct name, as among the very 

 few fossils found in the latter, Murchisonia boydi and Cyclonema sulcatum occur in 

 the Guelph. Megalomus canadensis, the most common species, and Trimerella 

 grandis are found in the Niagara. The characteristic fossils are Pentamerus occi- 

 dentalis, Murchisonia bivittata, M. longispira, Subulites ventricosus, Phurotomaria solar- 

 ioides, and Dinobolus galtensis. 



5 



