686 SCHUCHERT AND TWENHOFEL ORDOVICIC-SILURIC SECTION 



MiNGAN Islands Succession 



GANADIG SYSTEM. BEEKMANTOWN SERIES 



Bomaine formation — thickness and general characteristics. — In the re- 

 gion of the Mingan Islands the Beekmantown deposits are seen at various 

 places between Saint John Eiver eastward to Sainte Genevieve Island, a 

 distance of about 55 miles. The thickness of the Beekmantown, which 

 will hereafter in this paper be called the Romaine formation (from the 

 island of the same name at the month of Eomaine Elver), is probably 

 not less than 290 feet and not more than 365 feet. Logan estimated it 

 at not over 250 feet, but he excluded about 90 feet that are herein in- 

 cluded. These deposits consist of dolomites and magnesian limestones 

 with thin bands of shale or shaly magnesian. limestone. 



Romaine formation, Division A^. — In the basal portion the dolomites 

 are more granular than in the higher strata and locally are greatly dis- 

 turbed by the diagenetic changes that have taken place during or shortly 

 after the time of deposition. The rock in such places appears as if 

 kneaded, and has many geodes, nodules, and patches of yellowish white 

 chert and yellowish white calcspar. These phenomena have been de- 

 scribed by Logan, who states that they "suggest the idea that they may 

 be the effect of ancient springs, which rise to the surface through the yet 

 unconsolidated sediment, washing away the finer particles, and disturbing 

 and confusing the arrangement of the strata'^ (Logan^ 1863, page 121). 

 Similar phenomena have been seen elsewhere in Beekmantown deposits 

 and are thought to be produced by the dolomitization of the yet uncon- 

 solidated limestones. 



The Eomaine A^ beds undoubtedly rest directly on the Laurentian, 

 although an actual contact has not been seen. The oldest observed strata 

 are on Eomaine Island, and about one-eighth of a mile northward occurs 

 the Laurentian granite, the interval being concealed by a shallow channel 

 that is rapidly being filled in by sands. The strata concealed here are 

 estimated not to exceed 30 feet. It is probable that in this concealed 

 zone occur the strata described by Logan as follows : 



"At the east born of Pillage Bay, on the main coast, opposite to Hunter's 

 Island of the Mingan group, a white sandstone about eight feet thick has been 

 met with resting on Laurentian gneiss. The position of this sandstone, and 

 the moderate dip of the palaeozoic strata in that neighborhood, would bring it 

 to within two feet of the nearest strata of the Calciferous formation" (1863, 

 page 287). 



The Eomaine Island section exposes at the base a shaly limestone not 

 over 3 feet in thickness, followed by a mottled dark gray, coarsely crystal- 



