166 Microscopic Structure of Canadian Limestones. 



disconnected, but in a vertical section they are seen to rest upon; 

 one another, and sometimes to be very closely packed, as in Fig- 

 3, in which translucent fragments of crinoids are seen to be 



Fig. 3. — Trenton Limestone, Montreal, vertical section T (10 diams.y 



packed in broken corals, chiefly Ptilodictya, and the irregularity 

 of the planes of deposition is marked by two slender bands of fine 

 black earthy and organic slime. 



The beds of this remarkable organic limestone are usually very 

 uneven on the surface — the smaller beds very much so ; and on 

 these surfaces there often appear quantities of Monticulipora and 

 Ptilodictya in a perfect state, as well as occasionally Brachiopoda, 

 Orthoceratites and Trilobites. Between the beds occurs a black shaly 

 material consisting principally of clay and fine sand, darkened by 

 carbonaceous matter, a.nd containing more or less of fragments of 

 shells and corals. The beds of organic fragments now constituting 

 the gray limestone, must have been drifted over the bottom by 

 strong and apparently somewhat irregular currents, in which in 

 -very favorable spot corals fixed themselves and grew. The black 

 shale appears to have settled in the form of fine mud, which often 

 coats over, as with a varnish, the surfaces of the limestone and the 

 fossils lying on them ; and which has usually only partially filled 

 up the depressions of the surface, previous to the deposition of & 

 new bed of the grey limestone. In the upper part of the Trenton 

 formation at Montreal, the earthy matter so far prevails that the 

 'imestone becomes black and compact, and is interstratified with 

 much shale, but it still contains numerous organic fragments, 

 which in some beds become predominant. 



The Trenton and its associated limestones are widely distributed 

 rocks. Their outcrop runs from Quebec along the north shore of 

 the St. Lawrence to Montreal — then southward through the valley 

 of Lake Champlain into New York, where it skirts the Azoic 

 region of the Adirondack, and returning northward along tke 



